Though Oswalt said after the game Thursday he wouldn't have an MRI until Monday, Phillies general manager Ruben Amaro Jr. told ESPN.com's Jayson Stark he would be checked out Friday and that the team expects to get an update on his condition Friday night or Saturday.
Oswalt gave up four runs on five hits in the 12-2 loss to the St. Louis Cardinals and departed trailing 4-0 with tightness in his lower back. It was his shortest stint since a two-inning effort Sept. 10, 2009.
Oswalt gave up a home run in the first to Jon Jay. He then allowed three successive singles to begin the second inning.
The 33-year-old Oswalt was on the disabled list from April 27 to May 16 with lower back inflammation. A three-time NL All-Star, Oswalt has won 10 or more games in nine of his 10 seasons. He threw 37 pitches on Thursday -- 26 for strikes.
Information from The Associated Press was used in this report.