In the big leagues everyone is hurt, it's not about staying healthy as much as its about being able to manage the injuries when they happen and some injuries are severe enough that it's not even possible to do so.
He's only missed substantial time once (2013 with the collar bone) and played very very well even with his limited calf last season. He's got nothing to prove, he's arguably the most efficient QB in NFL history bad calf last year and all.
Favre had the steak but was he consistently healthy? Hell no! He played with many an injury, shoulder contusion, broken thumb on the throwing hand, etc. I didn't seen anyone questioning his "ability" to stay healthy.