The Red Sox kept moving on deadline day, and the latest deal sends one of their former blue-chip prospects out the door.
Boston traded Marcelo Mayer to the Giants for left-handed reliever Erik Miller, according to Jeff Passan. The Boston Globe’s Alex Speier reported that outfield prospect Carlos Gutierrez is also headed to Boston in the swap.
Mayer’s fall from centerpiece prospect to trade chip has been a quick one. The Red Sox took him fourth overall in the 2021 draft, and he climbed fast through the system, reaching Double-A Portland in 2023 and Triple-A Worcester in 2025 before getting the call to the majors in May 2025. He was once part of Boston’s “Big Three” alongside Roman Anthony and Kristian Campbell.
The big league results never really matched the pedigree. In 364 plate appearances for Boston, Mayer hit .223 with a .625 OPS. This season, he was at .220 with a .594 OPS before landing on the injured list in late June.
Miller gives Boston a proven arm from the left side. The 6-foot-5 reliever has appeared in 147 major league games since debuting with San Francisco in 2024, posting a 3.05 ERA and 1.362 WHIP with 154 strikeouts in 130 innings. This year, he has a 2.76 ERA and 1.378 WHIP.
Gutierrez adds another prospect to the Red Sox system. He ranked 17th in the Giants organization and was batting .271 with an .826 OPS for High-A Eugene this season.
The move came after a much larger transaction earlier Monday, when Boston sent pitchers Anthony Eyanson and Kyson Witherspoon, outfielder Enddy Azocar, catcher Carlos Narvaez and a player to be named later to the Orioles for Adley Rutschman. The Red Sox also dealt pitcher Tyler Uberstine to the Braves for outfielder Eli White.
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