Garrett was optioned to Triple-A Jacksonville on Tuesday, Christina De Nicola of MLB.com reports.
Garrett started Game 2 of Tuesday's doubleheader against the Rangers and gave up two runs over 4.2 innings, and he won't remain with the big-league club. The 25-year-old has a 3.68 ERA, 1.19 WHIP and 75:19 K:BB across 73.1 frames this year, so he figures to rejoin the Marlins at some point down the stretch.