The two brothers will face each other in what has been dubbed the 'Manning Bowl' when the Denver Broncos take on the Giants this weekend, a scenario that both feared would be unlikely to happen when Peyton underwent neck surgery in 2011.
However, any doubts over whether the four-time league MVP could return to his best were quashed when he threw 37 touchdown passes last season and started the current campaign with an NFL record-equalling seven touchdown passes in one match.
"I knew it would get better with time, but after several surgeries and you're dealing with nerves, you didn't know how quickly [he would recover]. That was the scary thing for him - he didn't know how quickly it would get better," Eli told ESPN.
"It's one thing throwing, and just dropping back and throwing, then avoiding the rush and throwing in awkward positions, and you can't step into it. Can you get the same strength on the throw that you normally can to be successful?
"He's obviously back and full throttle. After seven touchdowns, there isn't any doubt. And maybe there wasn't after 40 touchdowns last year. You know, he never ceases to amaze me. All you can do now is hope he has gotten it all out of his system."
The Manning brothers have met each other twice in the NFL before, with Peyton coming out on top on both occasions.