The Orioles had hoped to avoid putting Mountcastle on the IL, but manager Brandon Hyde said Mountcastle tried to swing before Wednesday's series finale at Houston and it "didn't go as well as we were hoping."
Mountcastle had missed the past six games with the injury. The move is retroactive to Sunday.
"It was going really well with his progression, and he was feeling better," Hyde said. "He tried to swing the other day, and he was a little bit sore. We were hoping that this session [Wednesday] was going to be an improvement from that last one, and it just wasn't."
Hyde said the Orioles hope Mountcastle will be ready to return as soon as he is eligible to come off the injured list.
Mountcastle is hitting .269 with 18 home runs and 67 RBIs for the American League East leaders.
Outfielder Ryan McKenna was recalled from Triple-A Norfolk to take his spot on the roster.