Pietrus was injured in the second quarter when he banged into the Pistons' Jason Maxiell and landed awkwardly on his back and arm. X-rays taken after the game showed no structural damage, but an MRI on Tuesday revealed the fracture.
"I don't think we'll know a real good timetable until we get home, but it's certainly going to be a significant amount of time," coach Stan Van Gundy said on Wednesday before Orlando played the Chicago Bulls.
One of the Magic's main 3-point threats, Pietrus is averaging 11.7 points. But he's had trouble staying healthy after the Magic signed him away from Golden State in the offseason.
Pietrus was appearing in his fifth straight game on Monday after missing 10 with a torn ligament in his right thumb.
"We've been sort of easing him back in and now he gets hurt again," Van Gundy said. "We've played so long without him now that, unfortunately, I think we're used to it. That doesn't mean it's good for us. ...
"He's probably the most athletic guy on our team outside of Dwight [Howard], so it's a big loss."