Mountcastle (foot) will begin a rehab assignment with Triple-A Norfolk on Tuesday, Jake Rill of MLB.com reports.
Mountcastle has been steadily progressing in his recovery from a left foot fracture that he sustained in mid-April, and he's ready to take the next step by embarking on a rehab assignment. He'll likely need to play in a few minor-league games before being cleared to return, but he is tracking toward being back with the Orioles by early-to-mid August.