IMO if Brock plays half decent or doesn't lose the game Manning might not play another snap in the NFL.
He's just done. The guy probably has 14 years of NFL experience telling him 'that guy's open, let it rip', while he also has 1.5 years of experience telling him 'you can't make that throw anymore'.
I also don't think it's all his arm. Mentally he's second guessing himself and whenever he gets hit it seems to take him weeks to recover, which add up when you participate in weekly car crashes.
Anyway, for you Bears fans you would have been better off playing against Manning. There's less tape on Brock and physically he can do a lot more in terms of arm strength and mobility. Personally as a non-fan of both teams that game just got a lot more interesting.