Garrett was sent to Louisville on May 7 in a bid to limit his innings this season. The 25-year-old made one start at Triple-A, striking out all six batters he faced in two innings.
Garrett posted a 3-2 mark with a 4.25 ERA in six starts this season for Cincinnati.
In a corresponding move, the Reds optioned right-handed reliever Barrett Astin to the Bats. The 25-year-old recorded a 6.75 ERA without notching a decision in six relief appearances this season.