I think the 1972 Dolphins would argue that, though. Starting QB Bob Griese misses most of the season with an injury, but the Dolphins still go 14-0 and win the Super Bowl. More often than not you're right, though. It's modern era football though. STL did it with Warner, but that was a preseason injury and resulted in a HOF caliber QB replacing a journeyman. Same thing with New England. You could get away with it if you are a dominant defensive team that hides the QB anyway, but if you rely on your QB for anything your backup isn't going to help. A journeyman QB might help an already decent team hold onto a playoff spot, but that's about it.