De Los Santos was recalled from Triple-A Lehigh Valley on Wednesday.
De Los Santos will provide length out of Philadelphia's bullpen after Drew Anderson, who was sent to the minors in a corresponding move, was used for four innings in Tuesday's loss. The 23-year-old De Los Santos owns a 2.81 ERA, 1.12 WHIP and 24:5 K:BB in three starts (16 innings) for Lehigh Valley this season.