The Red Sox are already down three key relievers and they just lost a fourth.
Right-hander Koji Uehara was placed on the 15-day disabled list with a right pectoral strain, the team announced Wednesday.
Uehara left Tuesday's game against the Giants after throwing just seven pitches in the ninth inning. He struck out Brandon Belt before exiting. He was replaced by Tommy Layne.
Boston is without injured relievers Craig Kimbrel, Junichi Tazawa and Carson Smith.
Right-handed pitcher Noe Ramirez was recalled to take Uehara's spot on the 25-man roster.
In 39 games this season, the 41-year-old Uehara has a 4.