Wells was claimed off waivers from the Chicago White Sox on Thursday.
Young batted .261 with eight home runs and 31 RBIs in 80 games. The first overall pick in the 2003 draft signed a one-year deal with Philadelphia in the offseason. He has a career .283 average over eight seasons with four teams.
"Honestly, I think that Delmon is a better offensive performer than he performed for us," Phillies general manager Ruben Amaro Jr. said before the team's series opener at Washington. "He just didn't do it enough, or consistently enough for us to have him continue to play."
Wells, who has been part of five organizations this season, will come off the bench in his Philadelphia debut. The 28-year-old was previously with the Mariners, Blue Jays and A's, along with the White Sox.