Packers coach Mike Sherman wouldn't delve into Pederson's long-term prognosis, but the injury led Sherman to ask for O'Sullivan to be included in Monday's big trade.
"I talked to Doug and I'm not sure how long he'll be out, but he's hurting still pretty bad today," offensive coordinator Tom Rossley said. "They said he had a broken rib on the back of his side. I'm not a doctor, but it's going to be awhile."
With Pederson likely out of action for the foreseeable future and Favre likely to sit out Wednesday's workout (the Packers don't practice on Tuesday), third-year quarterback Nall will run with the first-team offense and O'Sullivan will be thrown into the fray.
Assuming Favre starts Monday night's key game against visiting Tennessee, Nall would be the backup and O'Sullivan would be the emergency third quarterback. Working in O'Sullivan's favor is the fact the Saints run a similar offense to the Packers. His offensive coordinator there, Mike McCarthy, was quarterbacks coach in Green Bay for a season.