De Los Santos was recalled from Triple-A Lehigh Valley on Monday.
The 25-year-old was sent down Saturday, but he'll rejoin the Phillies' bullpen after Vince Velasquez (finger) was placed on the 10-day IL on Monday. De Los Santos has recorded a 6.75 ERA and 1.75 WHIP in 16 innings across 15 relief appearances this year.