Mountcastle isn't starting Wednesday against the Cardinals due to left wrist soreness, Roch Kubatko of MASNSports.com reports.
The Orioles didn't initially provide a reason for scratching the 25-year-old from Wednesday's lineup, but he's considered day-to-day with a sore wrist. If Mountcastle is forced to miss additional time, Trey Mancini and Tyler Nevin would likely see an uptick in playing time at first base.