Can we know how good a defense is after it's faced Matt Moore? How about Jimmy Clausen? Tyler Thigpen? Could the 2010 Bears defensive dominance be a lot like the 2007 Seahawks defensive "dominance?"
Seahawks fans know all too well how a run of bad quarterbacks can make an average defense look like world beaters. Though there are no Alex Smiths or A.J. Feeleys in the mix for the Bears, they have faced quite a few quarterbacks of lacking or questionable ability.
In week one, the Bears faced Matthew Stafford, knocked him out of the game, and then Shaun Hill after Hill subbed in. In week five, the Bears faced Jimmy Clausen, knocked him out of the game, and then Matt Moore after Moore subbed in. In week 10, the Bears beat down old man Brett Favre. The following week Chicago matched against Tyler Thigpen and the Dolphins. It was Thigpen's first significant regular season action since 2008. In week 13, the Bears faced Drew Stanton. I like Stanton, but it's hard to know exactly how good he is. Everyone loved Derek Anderson in 2007, but Anderson fell to Earth like so much space garbage. In week 15, Chicago beat down old man Brett Favre again, knocked him out of the game, and faced Joe Webb after Webb subbed in. It was the rookie’s first meaningful action of his career. In week 16, the Bears faced Mark Sanchez. Sanchez had some success, and Sanchez is among the worst starting quarterbacks in the NFL.
Seahawks fans did not know for sure that the run of jobber quarterbacks the team faced in 2007 was a big part of the defense playing so well, but it seems obvious in retrospect. Trent Dilfer, Jeff Garcia, Marc Bulger, Gus Frerotte and A.J. Feeley were all at or near the end of their careers. Matt Moore and Troy Smith were rookies then and low level backups now. Alex Smith and Rex Grossman are busts.
Likewise, maybe someday Stafford, Clausen, Webb, Thigpen, Sanchez and Stanton will be good quarterbacks, but none are good quarterbacks now and none have exhibited much to make me believe they are about to become good quarterbacks. The big picture is that it's hard to know how good a defense is when it's facing rookies, the soon to be retired, journeymen and backups. Advanced stats adjust for quality of opponent, but it's hard to account for the precise difference between a starter and a backup, a game starter and the player that substitutes in after injury, and to account for the wildly variable performance of bad quarterbacks.
Perhaps the single most damaging thing a quarterback can do, throw an interception, is infrequent and unpredictable and can bunch up against some opponents and not happen at all against others. We know from a larger sample which quarterbacks are interception prone, but the most interception prone quarterback in football, Eli Manning, only threw an interception in 4.6% of pass attempts. Over 34 attempts, a typical game for Manning, that still only yields about 1.5 interceptions. And in practice, sometimes Manning avoided interceptions all together as he did against the Bears fifth ranked pass defense, and sometimes he exploded for five picks in 83 attempts, as he did over two games against the Cowboys 28th ranked pass defense.
Facing bad quarterbacks is like playing with loaded dice. You don't win every roll, but probability is switched in your favor. The Bears faced quite a few truly bad quarterbacks, and in seven games, those quarterbacks and the teams they helmed accounted for 19 of the Bears 35 turnovers on defense. Maybe Chicago just exploited the competition they faced, and maybe the competition they faced made Chicago look like a much better defense than it actually is.