The Phillies hope that Park will be ready for the postseason, but if he's out three weeks, it could put his availability for the Division Series in jeopardy.
Phillies assistant general manager Scott Proefrock says Park will report to Clearwater, Fla., for treatment. Park, who is 3-3 with a 4.43 ERA in 45 games this year, left Wednesday's win over Washington after pitching a scoreless eighth inning.
Park was demoted to the bullpen in May after a poor start in the rotation. He has a 2.52 ERA in 38 relief appearances.
The NL East-leading Phillies made the announcement about Park's injury before Thursday's game against the Nationals.
ESPN.com senior baseball writer Jayson Stark and the Associated Press contributed to this story.