How to keep your starters healthy? MLB teams are shifting to six-man rotations, because more arms means more rest.

Last Monday, the Sox added a sixth when they reached an agreement with righthander Walker Buehler on a one-year, $21.05 million deal. The former Dodgers ace’s return from Tommy John surgery in 2024 (1-6, 5.38 ERA, 19 percent strikeout rate) fell far short of his pre-surgery standards, but he offered a reminder of his top-of-the-rotation upside during the postseason.

The acquisition points to two developments in modern baseball. First, teams need to insure themselves from the growing risk of injuries, particularly season-derailing surgeries.

“We’ve seen cases when guys get hurt and we need to plug holes, and what happens if there’s a significant drop-off between those in our rotation and the depth that we’ll need to call upon,” said Red Sox chief baseball officer Craig Breslow. “Given the starters we have, and given the younger guys that have come up and shown significant upside — whether that be [Richard] Fitts or [Quinn] Priester or [Hunter] Dobbins or [Cooper] Criswell — I feel like the depth we have amassed will enable us to weather performance regression, injuries, etc.”

Yet even beyond the idea of six solid starters as an insurance policy, teams are redefining the schedule of their rotation members. Managers typically say they’ll deploy their starters on “normal rest” or give them “an extra day.” Yet those terms, as traditionally employed, are no longer accurate.

For decades, “normal rest” meant four days of rest between starts while an extra day meant a fifth. As recently as 2016, there were roughly 1½ starts on four days of rest for every start on five.

That’s no longer the prevailing convention. Starts on five days of rest have skyrocketed, and there were roughly 1.3 times as many starts on five days of rest as on four. Starts on six days of rest have increased by 57 percent in the same time.

The trend is unlikely to abate any time soon in baseball’s Icarus Era, where pitchers constantly chase the solar sirens of velocity and spin while flying with wax wings.

Intuitively, the easiest way to limit the risk of a major injury is by limiting the frequency with which they throw. Additionally, the best way to withstand a major rotation injury is by having enough healthy major league starters to endure a pitcher’s prolonged absence.

A six-man rotation seemingly accomplishes both of those goals, resulting in growing plans to employ the tactic — particularly by teams who have starters coming from Japan’s Nippon Professional Baseball, where pitchers typically throw once a week.

The Angels employed a six-man rotation at times when Shohei Ohtani was in their employ, but have moved away from it. The Dodgers, who had the fewest starts in the majors on four days of rest last season (11), plan to use one in part to manage the health and schedule of Ohtani as he returns to pitching. The Mets also plan on using a six-man rotation outside of instances where an unusual cluster of offdays makes such an approach impractical. The Orioles have used a six-man rotation at times in recent years, and may consider employing one in 2025 to help Tomoyuki Sugano as he acclimates to MLB after a decorated career in Japan.

The Red Sox had more starts on four days of rest (68) than any other team last season, and were one of just six teams to have only five starters make 10 or more starts. But they are willing to consider a six-man, and other measures in an ­effort to make their staff healthier, deeper, and more effective.

“Being creative — a six-man rotation, openers — everything needs to be on the table,” said Breslow.

There are tradeoffs to a six-man rotation. Foremost, it means one fewer bullpen arm — thus increasing the need for longer starts and/or multi-inning relievers. And more rest doesn’t necessarily equate to better health or better performance.

Since 2021, there are 189 instances of pitchers who had at least 10 starts on four days of rest and 10 starts on five days. Ninety were better on the four, versus 98 one the five and one whose ERA was the same.

In the case of the Sox, Houck, Crawford, and especially Bello (3.56 ERA on four days of rest; 7.16 ERA on five days) were all notably better with less rest last year. The Mariners and their MLB-rest rotation last season ranked fifth in the number of starts on four days of rest (62).

Meanwhile, even though the Dodgers had the fewest starts on four days of rest, they lost more games to injury (1,834) than any other team according to Baseball Prospectus, and had just two pitchers log 100 innings.

“Objectively, it’s really difficult to draw any kind of hard and fast rules [about how much rest maximizes performance], so instead we rely on a larger data set, technology that helps us track movement, pitch shapes, velocity, track all of those things and put the pieces together,” said Breslow.

As of Thursday, there had been 20 big league contracts for free agent starters, including 12 multiyear deals topped by Max Fried’s eight years and $218 million with the Yankees. By contrast, there had been just 13 big league deals for relievers, and three multi-year deals headed by the relatively modest two-year, $22 million commitment by the Dodgers to bring back Blake Treinen.

But a run on relievers is coming, and when it does, no one is expected to garner more suitors than Tanner Scott. The lefthander had a 1.75 ERA and 29 percent strikeout rate in 72 innings while serving as a bullpen Swiss Army knife who could close, set up, and deliver 3-6 outs in a game.

“He is one of the most impactful lefthanded relievers you’re going to find,” said Marlins president of baseball operations Peter Bendix, who traded Scott to the Padres last season. “He wants the ball every day. He is a bulldog on the mound, he’s afraid of nobody, and he throws 100 [miles per hour] with a wipeout slider.”

Most forecasts suggest a four-year deal for Scott. Might he surpass that?

In the 1990s and early 2000s, teams rarely signed relievers to four-year deals and never inked five-year pacts. That changed when lefty B.J. Ryan agreed to a record-setting five-year, $47 million deal with the Blue Jays after the 2005 campaign.

J.P. Ricciardi, then Toronto’s general manager, took the plunge with eyes wide open.

“The attrition rate of relievers is scary. Pitchers, they’re like racehorses. You never know when they’re going to go down,” said Ricciardi. “[But] the bottom line was we weren’t getting him unless we went to five years. I took the five years banking on he would be good for maybe four of those.”

Ricciardi got about half of that hoped-for return. Ryan was an All-Star in his first year, blew out in the second, and was very good in the third. His velocity and effectiveness cratered in the fourth, leading the Blue Jays to release him with more than a year left on his deal. Ryan never pitched again in the big leagues.

No other free agent reliever received a five-year deal until Aroldis Chapman (Yankees) and Kenley Jansen (Dodgers) after the 2016 season. More recently, each of the past two offseasons featured a pair of five-year deals for free agent relievers, with Edwin Díaz (Mets) and Robert Suarez (Padres) getting deals of that length after the 2022 season, and Josh Hader (Astros) and Yuki Matsui (Padres) doing the same last winter.

There’s some logic to teams assuming more risk. The workloads of elite relievers have remained steady over time. Elite starters, on the other hand, are seeing their workloads dwindle, leaving teams scrambling for late-innings difference-makers.

“These are all individual decisions,” said Padres GM A.J. Preller. “You understand the nature of that job, but when you look at individual factors — delivery, strikes, how well guys are in shape, effort level — I think there’s different bets. Some guys are relievers you wouldn’t want to bet more than a year on. Some guys are relievers that you want for multi-years.”

Scott certainly falls in the latter camp. Is it possible he could get five years?

“Somebody’s always going to go do the extra thing,” said Ricciardi. “It’s supply and demand.”

The Red Sox’ agreement with Walker Buehler came just days after the team’s two-year, $18.25 million deal with lefthanded starter Patrick Sandoval became official.

Sandoval, who is recovering from Tommy John surgery, won’t be ready until the second half of the season, if at all in 2025. But with Lucas Giolito and Buehler eligible for free agency after this season, the Sox opted to take a proactive approach to building depth for 2026, while hoping that changes to usage (likely fewer fastballs, and more emphasis on an elite changeup and quality breaking balls) could allow them to hit on some upside for a pitcher who had a 2.91 ERA in 27 starts in 2022.

“We largely felt like this was an opportunity in some ways to get ahead of where we’ve seen the pitching market go,” said Red Sox chief baseball officer Craig Breslow. “We identified Sandoval as someone we felt like we could provide with some development opportunities, could tweak the pitch usage a little bit, and could get him back to where he was a couple years ago as a dominant lefthanded starter.”

Breslow said the Sox’ signing of Sandoval wouldn’t impair the team’s ability to address more pressing needs for 2025.

“We’ve been very outspoken about making moves that impact our 2025 team,” said Breslow. “But when we have the opportunity to also improve the future outlook in a way that we felt like didn’t detract from our ability to improve 2025, it made sense to do so.”

Other Red Sox notes:

▪ The willingness of Sandoval and Buehler to come to Boston on short-term deals while trying to prove they can be both healthy and dominant on the way to a next big contract is notable for a couple of reasons. First, it suggests the Red Sox are willing to pay handsomely for pitchers on relatively short-term deals. Secondly, it suggests that pitchers are open to the idea that they can thrive even while pitching for the Sox at Fenway Park — not always a given.

▪ While Breslow wouldn’t comment on the signing of Buehler before its official announcement on Saturday morning, he did acknowledge that the team’s roster-building focus has shifted primarily to the bullpen and adding righthanded bats.

“Those are probably the big rocks,” said Breslow. “There’s always the chance that there’s some opportunity that comes across, or that we’re able to uncover that isn’t one we’re anticipating, but we’ll likely shift focus to rounding out the bullpen and balancing out the lineup.”

▪ Red Sox manager Alex Cora, in a radio interview in Puerto Rico shortly after the agreement with Buehler became public, identified the Sox’ top priority as a righthanded-hitting second baseman — a not-thinly veiled reference to his ongoing interest in bringing Alex Bregman to Boston.

â–ª Another Red Sox who might have interest in Bregman: Buehler. The two co-own a 3-year-old colt, March of Time, who won a race Friday at Santa Anita Park.

▪ The Red Sox showed some interest in free agent Gleyber Torres — who would have filled the righthanded-hitting second baseman profile — but never made an offer to the 28-year-old before he agreed Friday to a one-year, $15 million deal with the Tigers. The Sox were believed to have other priority targets ahead of Torres.

â–ª Throughout 2024, Kyle Teel joined Marcelo Mayer and Roman Anthony in regular conversation about what it would be like to take part in a duck boat parade following a title in Boston. That made being dealt to the White Sox as the headline prospect in the Garrett Crochet deal particularly hard to fathom.

“I really never thought that I was going to be traded,” said Teel.

Still, Teel said he needed just a couple of hours on Dec. 11 to move past the shock of being dealt, and now embraces the idea that he can form bonds within his new organization as Chicago tries to create a championship culture from the ground up.

“My mind is at, ‘How can I help the White Sox be the best they can be and how can we eventually win a World Series there?’ ” said Teel. “It’s a great opportunity. I’m going in there trying to help the organization get better and help create a winning culture.”

As for his former teammates, particularly close friends Mayer, Anthony, and Kristian Campbell?

“I see really, really big things for them in the future,” said Teel. “It was an honor to play with those guys every single day.”

▪ Yhoiker Fajardo, the 18-year-old pitcher whom the Red Sox acquired from the White Sox for lefthander Cam Booser, was described by an evaluator as being in “a Clay Holmes-type mold,” a 6-foot-3-inch righthander with a diving sinker (currently in the low 90s, but with room to get to the upper 90s with added strength and some physical tweaks) as well as a tight slider that ducks below bats with swing-and-miss potential. His 8-to-1 strikeout-to-walk ratio ranked fourth in the Dominican Summer League last year.

Happy birthday, Bill Lee. The Spaceman was almost surely ready to pitch in a game as he completed his 78th circumnavigation of the sun Saturday … Happy 26th birthday, Enmanuel Valdez. This month, when he was dealt to the Pirates for minor league pitcher Joe Vogatsky, Valdez joined Nick Yorke as the second Red Sox second baseman in the span of five months whom the Sox acquired under former chief baseball officer Chaim Bloom and subsequently traded under Breslow to a team run by former Red Sox GM Ben Cherington.

Alex Speier can be reached at [email protected]. Follow him @alexspeier.