The Nationals placed Garrett on the 15-day injured list Thursday with right biceps inflammation.
Garrett will be eligible to return for the Nationals' first game out of the All-Star break, but it's unclear if he'll be fully recovered from the injury by then. Prior to sustaining the injury, Garrett covered 7.2 innings out of the Washington bullpen and surrendered seven earned runs on 11 hits and seven walks. Joan Adon, who is scheduled to start Thursday's game in Philadelphia, was called up from Triple-A Rochester in a corresponding move.