It seems like that's the big question lately, with Peyton Manning doubtful for Sunday as he rehabilitates his neck. The football punditeratti say there is no other individual player so critical to his team's success than Manning, and they're probably correct.
Win Probability Added (WPA) can't do everything, but one of the things it can do extremely well is tell us exactly how much of every win or loss was due to one component of a team. In this case, it can tell us how many wins the Peyton Manning passing game can account for. Although we can't really separate Manning from his blockers and receivers, we can nail down a hard number for the Colts passing game as a whole, of which Manning has been the central fixture.
Since 2000 (as far back as my data goes), Manning played in 176 regular-season games and accumulated a total of 43.0 WPA, for an average of 3.8 WPA per season. This equates to 0.24 WPA per game, which means that Manning (and his passing offense) would give an otherwise perfectly average team a 74% chance of winning a game. In other words, he would take an 8-win team and make them an 11.8-win team.
To put Manning's numbers in perspective, Tom Brady's career WPA/G is 0.16, still extremely good, but not quite up in the Manningsphere. Drew Brees and Ben Roethlisberger have both averaged 0.14 WPA/G. Kerry Collins, who is likely replacing Manning at least for week 1, has averaged 0.01 WPA/G (since 2000). In 2010, including the playoffs, Aaron Rodgers averaged 0.23 WPA/G.
In that same period, the Colts have gone 125-51--a 0.710 winning percentage. That's not far off from the 0.740 winning percentage that WPA told us to expect. It also suggests that the Colts have been an otherwise below-average team, although not nearly as bad as many pundits suggest.
In terms of points, Manning totaled 1,617 Expected Points Added (EPA) over those 176 games since 2000. That's a 9.1 net point advantage per game. This means that Manning and his passing offense generated just over 9 points per game more than an average NFL passing game would have. It appears the odds makers agree, dropping the point spread in the upcoming Colts-Texans game by about that amount when Manning was declared doubtful.
How many wins has your favorite QB accounted for?