Garrett will start Game 2 of Sunday's doubleheader against the Phillies, Jordan McPherson of the Miami Herald reports.
The 23-year-old is serving as the 29th man for the twin bill, and he'll make his big-league debut in the nightcap. Garrett had a 3.34 ERA, 1.23 WHIP and 118:37 K:BB over 105 innings at High-A Jupiter last year.