His leg was fucked last year. I think he'll look more like 2013 Peyton than 2014 Peyton.
But: if I'm wrong, and the best he can do is game manage... they aren't going to be able to do a damn thing with him behind center, and it's probably time for him to retire.
Even if his arm is fucked and he has to be more of a game manager it does not mean the Broncos can't be successful. They just have to manage how many times he's throwing. He can't sit back and throw 50 times a game anymore, and by now good teams have already figured out how to play his offense a long time ago. Broncos can still be very successful if they have a good running game. They don't need a Seahawks type of running game, but a good solid running game will do wonders for Manning. That is of course if he checks his ego, and actually runs the ball/trusts his offensive coordinator.