Muller (shoulder) will begin a rehab assignment at Triple-A Las Vegas on Tuesday, Martin Gallegos of MLB.com reports.
Muller has been sidelined since the end of May due to tendinitis in his left shoulder, but he has now been given the go-ahead to test his shoulder in a game setting. The A's haven't specified how long they plan on keeping the 26-year-old in the minors, but he will likely require multiple rehab outings after being held out of action for nearly a month.