The 22-year-old Dominican Republic native was set to start the game against the New York Mets, making his major league debut. De Los Santos was 9-3 with a 1.89 ERA in 16 starts for the Iron Pigs this season.
When the Mets counter with righty Drew Gagnon, who is also appearing in his first big league game, it will be the first time a Phillies starter makes his debut against another starter in his first outing since Sept. 25, 1944, according to the Elias Sports Bureau. Back then, Charlie Ripple faced Chicago's Charlie Gassaway at Shibe Park.
To make room for De Los Santos on the 40-man roster, Philadelphia transferred Pedro Florimon to the 60-day disabled list. The versatile utility man has been idle since May 30 with a fractured right foot.
The club also promoted infielder Trevor Plouffe and optioned outfielder Dylan Cozens to Lehigh Valley. A veteran of eight seasons, Plouffe last played in the majors in 2017 and combined to hit .198 in 100 games for Oakland and Tampa Bay.
Left-hander Hoby Milner was designated for assignment.