The Arizona Cardinals are having an excellent season. At 8-1, they’ve got the NFL’s top record, and they’re fourth in our NFL Elo rankings with the best rating the franchise has had since 1977. In the most recent edition of our Elo-fueled simulations, they had the league’s second-best chance of winning Super Bowl XLIX.

But disaster struck the Cardinals on Sunday when Carson Palmer, their starting quarterback (and the recent recipient of a huge contract extension), injured his left knee early in the fourth quarter of the team’s win over the St. Louis Rams. Arizona announced Monday afternoon that Palmer suffered a torn ACL, which ends his season and hands the reins to backup quarterback Drew Stanton.

As I wrote last week (when Mark Sanchez assumed the Philadelphia Eagles’ starting QB job), there’s typically a noticeable decline in team performance — about 1.5 wins per 16 games — when a backup has to play. But in that same study, I also found little evidence the degree to which the starter is better than the backup — in terms of their respective career passing efficiency indices before the injury — changes the expected team decline on a case-by-case basis. This was good news for Philly and also would help Arizona, as Stanton’s career Adjusted Net Yards Per Attempt index (ANY/A+) is an abysmal 85 (100 is the NFL average), making him worse than the bottom bound of acceptable performance for a starter.

The latter finding doesn’t necessarily mean there’s no difference in ability across the league’s starters and across its backups. More likely, it’s evidence that for the majority of quarterbacks in the NFL, we simply don’t have a large enough sample of past data to know who’s actually good or bad — at least not enough to predict which ones will sustain their performance. Certainly Stanton’s track record tells us next to nothing about how good he is beyond the knowledge he’s a career backup. He’s thrown 280 career passes, but the majority of the damage to his career ANY/A+ was done in 51 horrid attempts for the Detroit Lions five years ago, aside from which his career ANY/A+ is an almost precisely average 99.

And amid the worries about whether Arizona can survive without Palmer, it’s also worth remembering that Palmer is one of the few QBs about whom we might have enough evidence to pass real judgment — and that evidence says he’s basically an average starting quarterback. In nearly 5,000 attempts, his career ANY/A+ is 104 — exactly the average ANY/A+ of injured starters who were replaced by backups in the data set I used last week.

The odds of Arizona becoming the first team to play a Super Bowl in their home building definitely decreased when Palmer was injured Sunday. But if the Cardinals were going to win the Super Bowl with Palmer as their starter, it would have been more because of their defense than their offense. Either way, this is another reminder that we can’t peg the damage of losing a starting QB any more precisely than when a generic backup takes over for a generic starter — particularly because Palmer’s career numbers say he typifies that generic starter.