In a move that still seems hard to believe, Craig Breslow traded Marcelo Mayer to the San Francisco Giants for reliever Erik Miller and an outfield prospect at the trade deadline. The longtime Boston Red Sox prospect went from "untouchable," in a sense, to gone in six months.
The Mayer for Miller (2.76 ERA, 45 strikeouts over 32.2 innings) trade makes the Red Sox better for their upcoming playoff push. But moving on from Mayer seems sudden, especially when the Red Sox declined to give him up for Arizona Diamondbacks second baseman Ketel Marte, a three-time All-Star, over the offseason.
But just as he struggled after his promotion, Mayer couldn't find his way offensively in 2026. He batted .220/.282/.312 over 70 games without the stellar defense he became known for in the minor leagues, and the Red Sox saw it as their chance to move on. Mayer didn't sound too banged up about the deal when speaking to reporters.
Marcelo Mayer is said to be enthused by the trade and a change of scenery out west. He’s excited about the new opportunity instead of being bitter about a lack of a long leash in Boston.
— Chris Cotillo (@ChrisCotillo) August 4, 2026
Some prospects don't pan out and that's an unfortunate reality of the way player development works in baseball. But the way Mayer's tenure went with Boston, or at least part of the way, is on the team.
The Red Sox were forced to call Mayer up before he was finished developing. In 2025, he'd played 43 games in Triple-A before Alex Bregman in late May sustained a quad strain. Mayer, despite spending nearly all of his minor league playing career as a shortstop, made his big league debut as a third baseman because Boston didn't have the depth to cover Bregman's injury. He didn't get to play his natural position with any degree of consistency because the Red Sox prioritized Trevor Story over him at shortstop, another mistake, given the way Story's defense has utterly degraded over the last two seasons.
Red Sox bungled Marcelo Mayer's development, call-up before major league struggles
Offensively, Mayer clearly wasn't ready to take the step to the majors. He batted .271/.347/.471 with five doubles, a triple and nine home runs in his 43 games, which is fine, but doesn't scream for a promotion.
Mayer had also never played a full season of professional baseball in his career due to injuries, which became a frequent and unfortunate staple for him. In 2022, he posted 91 games between Single-A and High-A, but that might as well be nothing compared to the length and stress of an MLB season.
Mayer needed more time to develop in the minor leagues, but that doesn't mean he didn't have enough time to show something worth keeping in Boston. But he batted .223/.278/.346 in his Red Sox career, couldn't hit right-handed pitching (.215/.277/.322) and his defense wasn't game-changing.
If Mayer had more time to grow in the minor leagues, maybe he would've ended up the top prospect the Red Sox expected he'd be. Instead, he joins Rafael Devers on the Giants and leaves fans to wonder what could've been if desperation hadn't taken hold last year in Boston.
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