The Red Sox placed Guerrero on the 15-day injured list Sunday, retroactive to Sept. 24, with a right shoulder strain.
Guerrero missed most of September with right shoulder inflammation before returning to the active roster Sept. 20. He's now dealing with more shoulder trouble, and it's uncertain if he might be able to return in October should the Red Sox advance deep in the playoffs. Guerrero has been a reliable arm for Boston this season, posting a 3.10 ERA and 1.11 WHIP with 18 holds across 40.2 innings (40 appearances).