Mountcastle is out of the lineup for Tuesday's game against the White Sox.
The 28-year-old is sitting for the fourth time in the past six contests, as Coby Mayo is operating as Baltimore's primary first baseman. Samuel Basallo and Tyler O'Neill have been picking up at-bats at designated hitter over the past week, which has pushed Mountcastle into a less consistent role. He hasn't helped his cause at the plate in September with a .194/.231/.222 slash line in nine games.