As reported by the Globe :
Celtics general manager Danny Ainge cited the NBA champions' short offseason as a reason Rondo and Perkins won't participate [in scrimmages against Team U.S.A.]. Perkins also has left shoulder concerns and might need surgery. "Perkins is not 100 percent, and he's definitely not playing," said Ainge.
From the Herald :
"I’m good with that decision," said Ainge, the Celtics director of basketball operations. "It’s been a long season."
Perkins faces the possibility of more surgery on his left shoulder.
The Celtics are comfortable that an earlier procedure on the shoulder met its desired goal. When Perkins injured it again in Game 4 of the Finals against the Lakers, it wasn’t a dislocation. Such instances simply tweak the area around the joint and, in this case, forced him to miss one game.
But the center may go in for another cut if it’s believed it will further improve his shoulder.
It's hard to tell if the "might need surgery" angle is coming from team sources, or whether it's just speculation from the media. Recently Perk said that the injury wasn't serious and he didn't expect to need further surgery, so I'm not exactly sure what to make of these reports. Let's hope there's nothing to them.