Patterson, placed on the non-football injury list Sunday, was diagnosed with a brain malformation last August after suffering a seizure during a training camp practice at Lehigh University.
Eagles head trainer Rick Burkholder said a piece of Patterson's cranium was taken off in January so neurosurgeon Dr. Robert Spetzler could remove the arteriovenous malformation (AVM), a tangle of veins and arteries that doctors determined caused the seizure. Burkholder said Spetzler determined Friday after examining an X-ray and CT scan that the area of Patterson's cranium that he operated on had not healed sufficiently to allow Patterson to return to football.
"He wasn't totally happy with it," Burkholder said. "He's concerned about some very small (potentially dangerous) situations he could get into in football."
The Eagles are scheduled to begin practice at Lehigh on Monday. Camp ends Aug. 14, and Burkholder said Patterson will be re-evaluated in a month but could potentially miss more than that.
"We thought he would be ready because today is six months since the surgery, (but) that's just what happens with healing," Burkholder said. "Everybody heals differently. I think we're all optimistic that he's going to play again. It's just not today."
Patterson, 28, is beginning his eighth year with the Eagles. He started 15 games last year after doctors determined he wouldn't be at risk putting off his surgery until after the season.
Burkholder said Patterson's condition is not considered career-threatening.
"Nobody's alarmed by it," he said.
Burkholder said Patterson will spend training camp at Lehigh working with the team's strength and conditioning coaches.
"His desire to play is great, but he's comfortable with the decision," Burkholder said.
Coach Andy Reid wouldn't say who will replace Patterson in the starting lineup. He said four tackles - Pro Bowl alternate Cullen Jenkins, 2010 starter Antonio Dixon, veteran Derek Landri and rookie first-round pick Fletcher Cox - will all share time with the starting defense during camp.
"We've got a bunch of players in that position," Reid said. "We took precautionary measures, and we made sure that (at that) position we didn't come up short. Even though you take Mike out of it, you're still looking at a strong position."