Red Sox trade Marcelo Mayer to Giants for Erik Miller

The trade deadline just claimed one of the bigger names in the Red Sox farm system. Ken Rosenthal broke the news that the Giants were closing in on Marcelo Mayer, and within minutes the whole industry was reporting the same shape of deal: Mayer to San Francisco, hard-throwing lefty Erik Miller back to Boston.

Ken Rosenthal

@Ken_Rosenthal

Giants close to acquiring Marcelo Mayer from Red Sox for reliever Erik Miller, sources tell me and Andrew Baggarly.

Jeff Passan and Mark Feinsand quickly confirmed the framework, with Passan noting the deal is still finalizing through medicals, standard language for a trade this size that hasn't officially cleared yet.

Jeff Passan

@JeffPassan

The Boston Red Sox are finalizing a trade to acquire left-handed reliever Erik Miller from the San Francisco Giants for infielder Marcelo Mayer, sources tell ESPN. Deal is still in medicals.

This one stings for Boston. Mayer was the 4th overall pick back in 2021 and spent years as one of the highest-regarded infield prospects in the sport. But 2026 was rough — he hit just.220 with three home runs across 228 plate appearances before a left forearm stress reaction landed him on the injured list in late June. A change of scenery is the bet the Giants are making.

Miller is the return, and he's not just a throw-in. The 28-year-old Stanford product, originally a 2019 Phillies draftee, has quietly been excellent out of the Giants pen this year — a 2.76 ERA over 32⅔ innings with 4 saves, 10 holds and a strikeout rate north of 32%. The walk rate is a bit high at nearly 16%, but he's got 3-and-a-half years of team control left, which fits a Red Sox club trying to lock down a bullpen for a playoff push after one of the hottest Julys in recent memory.

MLB Trade Rumors

@mlbtraderumors

#RedSox deal former top prospect to #SFGiants for hard-throwing lefty reliever t.co/rVdPe3JL0E

For Boston, it's a swap of long-term upside for short-term certainty — betting a former blue-chip prospect who's stalled out for a controllable, MLB-ready lefty arm right now. Once the medicals clear, both sides start figuring out how these guys fit their respective 2026 stretch runs.