Apostel was promoted to Triple-A Round Rock from Double-A Frisco on Monday.
The 22-year-old returned from the injured list for Frisco on July 28 after missing nearly a month with an unspecified injury, and he'll quickly move up to the highest level of the minors. Apostel posted a .236/.317/.419 slash line and 31.1 percent strikeout rate at Double-A this season, and he may need to improve upon those numbers at Triple-A to be in the mix for a big-league call-up down the stretch.