A lot of people are saying Peyton Manning doesn't play defense and the Colts defense is to blame for the decline in their performance (0-8 after going 10-6).
But at least someone at ESPN understands why the QB does influence the defensive statistics:
Brady managed only 198 yards passing, and Roethlisberger deserved some credit in containing the two-time NFL Most Valuable Player. Roethlisberger wasn't just the best offensive player at Heinz Field. He might have been the best defense.Roethlisberger's efficient effort allowed the Steelers to convert eight of their first 10 third downs. That kept Brady on the sideline as Pittsburgh dominated time of possession (39:22 to 20:38). In fact, Roethlisberger threw as many passes (50) as the Patriots had plays.
The Colts have always had a better offense and worse defense than people realize. The fact that the Colts have been able to drive the ball so well and Peyton has thrown so few interceptions has given the Colts defense time to breath and rest, good field position, and limited the number of possessions per game. That last effect is the easiest to quantify as we can compare how good the Colts defense has been since 2003 in scoring defense and scoring defense per possession.
From the last column you can see that every single year during the Colt's run of success the defense had a higher scoring defense rank than it's per possession rank, thanks apparently to the Colts offense keeping them off the field. It's harder to quantify (or demonstrate) the benefits from fresh legs, "playing with a lead," and good field position. We can try to estimate these effects by doing some statistical estimates of how, on average, an offense not turning the ball over as much and having more success with drives (in yards) will influence a defense and then seeing how the Colts compare to other teams after these factors have been eliminated.
That would take a lot of work but I might come back and do that if I have time.
Update: Bill Barnwell notices that QBs have an impact on defense:
The only thing that's really kept them afloat in 2011 has been the long fields provided to them by the New England offense; even when Tom Brady turns the ball over, it's usually been deep inside opposition territory. The 73 possessions the Patriots have faced have started with an average of 76 yards to go for a touchdown, the deepest starting field position in football.
Since he is a Patriots fan he seems unlikely to acknowledge that this observation applies to the Colts during their run of success.