Alex Bregman is off the free agent board after leaving Boston to sign a five-year, $175 million contract with the Cubs on Saturday.

Who will now play third base for the 2026 Red Sox?

Boston has had 23-year-old Marcelo Mayer working out at both third base and second base this offseason.

As a rookie last season, Mayer made 28 of his 35 starts at third base. His other seven starts came at second. He was promoted from Triple-A Worcester when Boston placed Bregman on the injured list May 24 with a right quad strain. The left-handed hitter started mostly at third base against right-handed starters when Bregman missed 43 games from May 24-July 11.

The sure-handed Mayer is considered Boston’s long-term shortstop. But chief baseball officer Craig Breslow has committed to keeping Trevor Story as his shortstop this season instead of moving the veteran to second base to open shortstop for Mayer immediately. That leaves Mayer as either Boston’s second baseman or third baseman depending on how the roster shakes out.

With Bregman gone, it’s looking more likely that Mayer will play third base.

The options on the free agent and trade markets are dwindling. The Red Sox could target free agent shortstop Bo Bichette to play second base. Meanwhile, free agent third baseman Eugenio Suárez, who hit 49 homers for the Diamondbacks and Mariners last year, remains available. But the 34-year-old would represent a significant downgrade from Bregman defensively. Suárez finished with minus-six defensive runs saved in 1,347 ⅔ innings at third base last year.

Mayer has the ability to play plus defense at third. He finished with 0 defensive runs saved in 248 ⅔ innings there last year. But the more reps he receives there, the better he should get. Most of his pro career has been spent at shortstop. He played just 48 ⅓ innings at third base in the minors compared to 2,254 innings at shortstop.

“It’s not easy going into an offseason kind of getting reps at every position,” Mayer said at Fenway Fest on Saturday. “I believe that every position requires different traits, different skills, different angles that you need to master. Obviously, I’m doing everything I can taking reps at third and second base and I feel really good at both. So wherever they need me is where I’m going to play. I’m going to do my best out there.”

He added that playing third base is completely different than playing second base.

“Second base, you’re doing everything backwards,” Mayer said. “Third base, you’re pretty much playing shortstop with less range, kind of quicker reflexes. So yeah, I think they’re just different skills that you need to hone in on to be able to be great at that position.”

Mayer spent the final 58 games of the 2025 season on the IL with a wrist injury that required surgery. He expects to be ready to fully participate in workouts once spring training begins.

“I’m pretty much doing full baseball activity, like a normal ramp-up, as I would for a regular season going into spring training,” Mayer said. “So I feel like I’m in a good spot.”

Mayer’s injury history is another concern if he replaces Bregman. It’s fair to question whether the Sox can rely on him to be available for the majority of a 162-game season.

The Red Sox asked him to put on weight this offseason to try to make him more durable. He has had issues staying healthy throughout his career so far, never playing more than 91 games in any season in the minors and majors.

“It was one of the main goals I set for myself going into the offseason,” Mayer said. “I weighed in at like 218 right now, which is by far the heaviest I’ve ever weighed in my life. I feel great, stronger and faster than ever. So I feel like my body’s in a really good spot.”

He’s up from 208 pounds at the end of last season.

“Moving well,” Red Sox manager Alex Cora said Saturday at Fenway Fest. “Actually, Trevor was surprised the way he’s moving. Fast. It seems like his offseasons, the last two or three, he’s always rehabbing or trying to catch up. Not this year. I had a conversation with him toward the end of the season and he basically said, ‘I’m ready, I’m ready.’ And we’ll see, we’ll see how it works out. But the kid, he’s a good baserunner, he’s a good defender, he can hit the ball out of the ballpark. Obviously there’s a few things offensively that he needs to improve, but that’s everybody. And I like the player. I like him a lot. I don’t think he’s afraid of this environment. He actually likes it. So just go out there and play in spring training.”

Another question mark is whether Mayer is ready to be an everyday starter who faces both righties and lefties?

Cora typically avoided batting Mayer against lefty starters and relievers last year, like he does with most all his young left-handed hitters. Mayer went 4-for-26 (.154) against southpaws while starting five games against them. He was 13-for-48 (.271) with a .300 on-base percentage, .458 slugging percentage and .758 OPS in 50 plate appearances against lefties for Triple-A Worcester before his promotion.

The Red Sox faced left-handed starters in 28% of games in 2025.

“I think he can play every day,” Breslow said at the GM Meetings in early November. “I certainly wouldn’t want to set limits on what he’s capable of doing. He hasn’t and that’s something we of course need to be mindful of.”

Cora said while discussing Mayer, “Facing lefties in spring training is going to make them better. If we don’t face them, we’ll figure out. … So just try to get them against lefties. Same with Roman (Anthony), same with Wilyer (Abreu), same with Jarren (Duran). That’s something that, like I said, we’ll talk with Bres and see where we’re at.”