Who has said he couldn't?
IMO, Rivera's decision to promote from within was as much about security as continuity. He knew he was on what was likely the hottest seat in the NFL coming into this season. Losing record, unthinkable late game meltdowns, new GM, etc. Promoting Shula meant that he would have two relatively inexperienced, unproven NFL coordinators. In short, he knew that if he got off to a slow start, there was simply no one on the staff to hand the reigns to in the event that the Panthers decided to cut the cord on his ass. If he'd hired a guy like Hugh Jackson to run the offense, there was a viable stop gap option available on his staff and that might spur the decision to just let Ron go and see if it was possible that we had the long-term answer to our coaching issue currently in house.