Tom Brady.
If he has a Manningesque drop off that could derail the Super Bowl 51 chances. See also Favre's 2009->2010 drop off.
Relying on a 39 year old QB to continue playing at or near MVP levels is certainly a risk.
We have been hearing this ever since Bradys knee injury and if anything he has been getting better, imo he has earned the benefit of the doubt and any drop off will be very slow and incremental. It will not be like Manning whose arm went dead a year after the spine fusions. Manning had significant nerve damage and that is something that is almost impossible to recover from. The analogy simply is not there. Bradys game is built on understanding defense's, accuracy, the quickest release in football, and exceptional pocket awareness and the ability to elude the pass rush without actually running the ball. And watching Brady play in recent years his arm actually looks stronger and his pocket awareness continues to get better and there has been no drop off in any area of his game. Will he last forever? Obviously not. Has he shown any sign of deterioration? None whatsoever.
My personal belief is that Brady is breaking the mold because of the way he cares for himself and that his focus on core strength, pliability, and flexibility (as week as his relentless work on his mechanics) is not only preserving his arm but Is actually making it better. And that same focus and hard work is what has allowed him to take the beatings he sometimes has to endure without breaking. There were a number of hits he took last season where he was hit from both sides at the same time and walked away from it when most QBs would have broken. I always believed Brady was being somewhat idealistic when he talked of playing until he was 45 and now I'm actually starting to believe that he could very well achieve that goal, especially if they can develop a really good OL over the next two seasons. And while I realize that's really pushing the limits I think that if anyone can do it Brady can, and if anyone has earned the benefit of the doubt it's also Brady.