Just how valuable is Eli Manning? That's the big question as the Giants and their quarterback approach an extension on his contract. And in at least one analysis, Manning is one of the most important quarterbacks in the NFL.

Paul Bessire and John Ewing of PredictionMachine.com ran 100,000 season simulations, 50,000 in which each NFL team had its starting quarterback and 50,000 in which each team had its backup for all 16 games. The results, as you might imagine, were that the probability of making the playoffs for nearly every team dropped with their backup behind center (only the Jaguars who have Chad Henne backing up Blake Bortles saw improved chances).

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The team with the second biggest drop? The Giants.

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According to the website, the Giants have a 31.9 percent chance of making the playoffs with Manning and a 5.2 percent chance of making the playoffs with backup Ryan Nassib. That's an 83.7 percent dropoff. Only the Falcons at 85.3 percent are more dependent on their starter.

Now it's hard even for a computer to project what Nassib would be able to do for a full season since he has played so sparingly. But it's also clear that the Giants' already minimal chances of succeeding in 2015 are tied to having Manning running the offense.

Other teams with big percentage differences between starter and backup are the 49ers (83.2 percent), Steelers (84.6) and Saints (80.3).

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The team with the largest actual differential in percentage points between starter and backup, by the way, is the Patriots with Tom Brady and Jimmy Garoppolo. But even with Garoppolo playing all 16 games (he'll currently start four, unless Brady's suspension is overturned or negotiated down), the Pats make the playoffs in 24.6 percent of the 50,000 simulations. That's a better percentage than 11 other teams with their starter.