Sizemore batted .216 with 10 doubles, two triples, two home runs and 15 RBIs for Boston this season. He was released by the Red Sox on June 18. The Phillies assigned him to Triple-A Lehigh Valley.
The Phillies also placed catcher Will Nieves on the 15-day disabled list because of a strained right quadriceps. The team reinstated outfielder Tony Gwynn Jr. from the restricted list. Gwynn's father, Hall of Famer Tony Gwynn, died last week.
Sizemore was an All-Star with Cleveland from 2006-08 and won two Gold Gloves in 2007-08. He's a career .266 hitter with 141 home runs, 473 RBIs and 139 stolen bases. Sizemore made it to the majors this year after injuries interrupted his career.