Mahler had been out since May 17 with a strained quadriceps muscle in his left leg. He is 1-0 this season with a 5.49 ERA in seven games, including three starts.
While on the disabled list, Mahler made a rehabilitation start for Nashville on Tuesday, losing to Tidewater, 2-0. Mahler pitched 7 1/3 innings, giving up six hits and two runs.
Although he was a part-time starter with the Reds, Mahler will return to the bullpen as a long reliever.