De Los Santos was designated for assignment by the Phillies on Sunday, Scott Lauber of The Philadelphia Inquirer reports.
The 24-year-old was optioned to the team's alternate training site Wednesday after a two-day stint in the majors, and he's now been cast off the 40-man roster. De Los Santos didn't appear in a game while up with the big club this season. He had a 7.36 ERA over 11 big-league innings last season.