Castano will serve as the 29th man for Friday's doubleheader against the Nationals and is expected to start Game 2, Jordan McPherson of the Miami Herald reports.
The 26-year-old will rejoin the active roster with Miami set to play it's final twin bill of the season. Castano has a 3.32 ERA, 1.58 WHIP and 9:9 K:BB through 19 innings this season and may return to the alternate site after the outing.