I have no idea what to expect from Manning. I was one of about 4 people who was not insanely excited about signing him because I knew he was barely able to grip a football a few months before. But he worked hard at getting better... although he didn't do very well in cold weather. Some people acted like his arm strength was fine, but I don't think he was the same guy at all.
Guys like Montana and Favre had one great season then regressed a bit at the end as age got to them. Elway won at that age but his team was loaded, Terrell Davis was running out of his mind. Bubby Brister won games with those teams.