The Red Sox recalled left-handed pitcher Roenis Elias from Triple-A Pawtucket on Thursday and optioned outfielder Bryce Brentz to Pawtucket.
The club also activated right-hander Sean O'Sullivan from the 15-day disabled list (knee) and outrighted him to Pawtucket.
Elias, 28, went 8-4 with a 3.72 ERA in 17 appearances (15 starts) for Pawtucket this season, though since May 18, he has gone 8-1 with a 3.05 ERA and .223 opponents' batting average.
The Red Sox acquired the 6-foot-1, 205-pound Cuban native last December from the Mariners. In two games (one start) earlier this season, Elias gave up 10 runs in 5 2/3 innings (15.