De Los Santos is slated to make his major-league debut Tuesday against the Mets, Tom Housenick the Lehigh Valley Iron Pigs reports.
De Los Santos will be added to the 40-man roster sometime ahead of Tuesday's matchup, after the Phillies shift Pedro Florimon (foot) to the 60-day disabled list. Philadelphia has elected to bring the 22-year-old prospect up from the minors to make a spot start, as they'll need a few spot starters following Monday's doubleheader. De Los Santos has been outstanding for Triple-A Lehigh Valley through 16 starts this season, posting a 1.89 ERA and 1.09 WHIP with an 87:31 K:BB over 95.1 innings.