Mountcastle was placed on the 10-day injured list Wednesday, retroactive to Sept. 17, with left AC joint inflammation, Jake Rill of MLB.com reports.
The Orioles had consistently said Mountcastle wouldn't need an IL stint for the shoulder injury he suffered a week ago, but he simply hasn't recovered as quickly as anticipated. He'll be eligible for activation Sept. 27, but it's not clear at this time whether he will be ready then.