Mountcastle went 2-for-3 with a pair of walks and two runs scored in Sunday's 8-6 loss to Boston.
The 24-year-old has produced four multi-hit efforts in 16 games in September, including twice in his last two contests. Mountcastle has enjoyed a solid rookie year, slashing .263/.316/.499 with 30 home runs, four stolen bases, 83 RBI and 72 runs scored through 535 plate appearances.