The 38-year-old broke the NFL records for passing touchdowns and throwing yards as he guided his offense to become the highest-scoring side in the history of the NFL.
Denver suffered an embarrassing loss to the Seattle Seahawks in the Super Bowl, but Fox has revealed that hasn't deterred Manning from wanting to get back to his best in the new season.
Fox told NFL Network: "I think so, we talk about that every day. Whether it is a football team, individual position or even an individual player.
"You know that's the goal to get better every day. He has worked very hard, he is in the best shape he has ever been in, all his numbers in our weight and conditioning program are all up from a year ago, even before that.
"There are areas he can improve in and he knows it and he will."
The Broncos face the Indianapolis Colts in the season opener on September 7.