Oswalt gave up four runs on five hits including a home run to Jon Jay, the second batter he faced, and the Phillies went on to lose the game 12-2. The three-times All-Star selection was visibly struggling to complete the second inning.
"I tried to stay in there as long as I could, but you can only pitch if you don't have any pain," Oswalt told reporters.
Oswalt (4-6) has made eight starts since coming off the 15-day disabled list earlier this season with inflammation in his back.
(Reporting by Mike Mouat in Windsor, Ontario)