An MRI last week showed mild bulging discs in Oswalt's back. He wants to be sure he can start a strengthening program and pitch this season without the risk of rupturing a disc and needing surgery.
If there are no surprises during the visit, Oswalt will likely start a series of injections to help strengthen his back.
He could return by August.
In other injury news, Phillies general manager Ruben Amaro Jr. said right-hander Ryan Madson may not be ready to return Monday, the first day he's eligible to come off the 15-day disabled list.
Madson has been dealing with a bruise on his right hand since getting struck with a line drive May 20 against the Rangers.
He will likely need a couple rehab assignments, Amaro said.
If Brad Lidge can get through a couple more bullpen sessions without any setbacks, he'll be ready to start a rehab assignment, Amaro said.
"Those are big ifs," Amaro said. "He hasn't pitched yet, so we'll see."
Joe Blanton is working out down in Clearwater, Fla., but he can't go more than a couple long-toss sessions without feeling tightness, limiting his progression.
"We're not going to get him out on the mound unless he's 100 percent and he's feeling like there's no stiffness or discomfort," Amaro said.