The Marlins placed McMillon on the 15-day injured list Thursday with right elbow tightness.
McMillon threw just two pitches and gave up a single before his injury forced him out of Wednesday's game against Pittsburgh, and he will now be forced to sit out until at least Sept. 27. The 26-year-old righty owns a 1.50 ERA and 1.00 WHIP across 12 innings in the majors this season, but given the fact Miami has already been eliminated from the postseason, the Marlins likely won't be in any rush to bring him back.