The Orioles placed Mountcastle on the 10-day injured list Monday with a sprained left wrist, Jacob Calvin Meyer of The Baltimore Sun reports.
Mountcastle initially suffered the injury while sliding into second base during last Thursday's game against the Astros and did not recover quickly enough to avoid a stint on the IL. He will be eligible to return Sept. 2, although it's unclear at this point whether he has a chance for activation on that date. Ryan O'Hearn should see most of the action at first base while Mountcastle is sidelined, with Eloy Jimenez picking up some playing time at designated hitter.