The Athletics reinstated Muller (shoulder) from the 15-day injured list Sunday.
Muller had been on the shelf since May 30 with left shoulder tendinitis but will rejoin Oakland after completing four rehab appearances with Triple-A Las Vegas. The southpaw covered three innings and tossed 61 pitches in his final outing for Las Vegas on Saturday, so the Athletics would presumably prefer not to use him in Sunday's series finale with the Orioles. Left-hander Jack O'Loughlin was optioned to Las Vegas in a corresponding move.