Rainey was returned to Triple-A Rochester following Thursday's doubleheader against the Mets, Jesse Dougherty of The Washington Post reports.
Rainey served as the 27th man Wednesday, and he remained in the big-league bullpen for Thursday's twin bill. The right-hander allowed two runs on three hits and no walks in a relief inning during Game 2 on Thursday, and he'll head back to the minors following his lackluster performance.