The problem is there aren't many solutions - at least that teams have found this season. Blitzing can be futile at times. Kelly said Manning and Patriots quarterback Tom Brady are the two best he's ever seen against the blitz.
"Very difficult when you see your quarterback playing at that level right now," Kelly said. "It's a tough challenge for us this week."
In the Eagles locker room, similar praise is shared. NFL players are often careful about providing bulletin-board material, so they offer banal platitudes leading up to the game.
But when asked about Manning, there's genuine respect. Manning, 37, has won four MVPs, been selected to 12 Pro Bowls, and has a Super Bowl ring. He's now playing as well as ever after missing a year because of a neck injury. He signed with the Broncos in 2012 after 14 seasons with the Colts.
Manning threw for 4,659 yards and 37 touchdowns in his first season with Denver. He opened 2013 with a seven-touchdown performance against the Baltimore Ravens, then bested his brother one week later in a 41-23 win over the Giants. On Monday night, only five of Manning's 37 passes fell incomplete.
What most impresses players is the way Manning plays at the line of scrimmage. His hand signals and theatrics there are all part of his mystique.
"He's going to come up there and do his show and try to get to you to show what you're doing," said Eagles cornerback Cary Williams, who twice played against Manning last season with the Ravens. "You have to try to stay as calm as possible in those situations, and don't give him anything. He can look at a stance, he can look at your eyes, and tell that you're coming. He's been around the game for so long, it's tough to figure this guy out."
Eagles defensive end Trent Cole, who was part of an Eagles defense that topped Manning in 2010, said Manning can see a defensive alignment and a blitz at the line of scrimmage and change it into a favorable play in a "good amount of time to get the ball off."
And even when defenses disguise their scheme, Manning is willing to wait. He gets to the line quickly, but doesn't call for the snap until he's ready.
"You have to try to give him different looks, but you have to make sure your disguise isn't a liability, that you're so far away from your work that you can't get to your work," Kelly said. "There's that cat-and-mouse game going on."
It's similar to how Kelly described playing against Philip Rivers in the Eagles' loss to the San Diego Chargers two weeks ago. If Rivers caused that much trouble, Sunday's game could get ugly unless the Eagles find a solution.
And it's not just Manning. Kelly lauded his weapons, including four pass-catchers with at least 190 receiving yards. Their top receiver is Demaryius Thomas, with 307 yards and two touchdowns. But Wes Welker is one of the NFL's top slot receivers and has four touchdowns, ascending tight end Julius Thomas has four touchdowns, and No. 2 wide receiver Eric Decker had eight catches for 133 yards in Monday's game.
"He has different guys out there capable of giving you 100 yards receiving," Williams said. "He has a legit team, a legit force. All we can do is go out there and play our game, and I look forward to it."