De Los Santos was recalled from Triple-A Lehigh Valley to start Sunday's game versus the Marlins.
De Los Santos will make the spot start with the Phillies in need of an additional starter due to Wednesday's doubleheader. The 23-year-old has allowed five runs over seven frames in the majors this season but has a 3.28 ERA, 1.16 WHIP and 44:13 K:BB across 35.2 innings at Lehigh Valley.