The move is retroactive to April 27, meaning Oswalt is eligible to return next Thursday.
"I've been a little bit tight since Arizona," Oswalt told CSNPhilly.com Friday afternoon. "It hasn't really gotten any better. I could probably pitch Saturday, but I couldn't promise anything on results. We'll give it a few more days to see how it feels."
To take his place, the Phillies recalled right-hander Scott Mathieson from Triple-A Lehigh Valley.
General manager Ruben Amaro Jr. said earlier Friday that Oswalt was feeling discomfort in his lower back. Oswalt initially felt the discomfort in his back during the Phillies' road trip to San Diego and Arizona. He last pitched against the Diamondbacks on April 26.
Amaro also said that second baseman Chase Utley is going to extended spring training and will play at least one inning in a game at Dunedin, Fla., on Saturday.
"It's a day-to-day process but he will be on the field to play," Amaro said. "He may play one inning or more if he feels like it."
Information from ESPN.com's Jerry Crasnick and The Associated Press was used in this report.