They brought up a pretty good stat during the game, even though all the commentators were clearly anti-Rex. They showed the comparison for the first 20 something games between Manning and Grossman. The stats were fairly similar, but the winning percentage was clearly in Rex's favor. The point was that even though he has been around for several years, this is truly his second year as a starter, and a contract year no less.
With all that said, I think we will know what we have after Week 3 (SD, KC, and Dallas).