Manager Brandon Hyde said Mountcastle is day to day.
"His hand is pretty sore," Hyde said. "Hopefully he feels better as the day goes on today. He's getting some treatment."
Mountcastle is hitting .253 with 15 homers and 55 RBIs for the Orioles, who are in the mix for an American League wild card after going 52-110 last season.
The Associated Press contributed to this report.