Sizemore will join the team for a three-game series against Washington before the All-Star break. Buchanan was the winning pitcher in a 9-1 victory at Milwaukee on Thursday. The move means he can start during the break and return to the Phillies when they need a fifth starter later this month.
Cliff Lee was moved to the 60-day disabled list to make room for Sizemore on the 40-man roster. Lee is expecting to return for a weekend series at Atlanta after the break.
Sizemore was a three-time All-Star for Cleveland from 2006-08. He began this season as Boston's starting center fielder before being released on June 18.