The Philadelphia Eagles beat the Houston Texans on Sunday, bringing their record to 6-2 — good for sole possession of first place in the NFC East. They also moved up two slots in our NFL Elo rankings; at the moment, their 1584 Elo rating ranks eighth among all NFL teams.But it wasn’t all good news for the Eagles on Sunday. The team’s starting quarterback, Nick Foles, suffered a “cracked” collarbone on the final play of the first quarter, necessitating an appearance by backup Mark Sanchez. While Sanchez played reasonably well in the victory, tossing a pair of touchdowns and posting a Total QBR of 53.4 (the NFL average for QBR is 50), there’s no small degree of trepidation among Eagles fans about the prospect of moving forward with Sanchez as the team’s starting QB for the next six to eight weeks.Anytime a team loses its starting quarterback, there’s a clear drop-off in performance. To quantify that decline, I went back and gathered data from every NFL game since 1978 in which a team was missing its regular starter at quarterback (defined as a QB who started at least 10 of 16 games for the team during the season). According to the Elo ratings going into those 1,585 games, which ostensibly assume the regular starter is playing, the starter-less teams should have expected to win 46 percent of the time. In actuality, they only won at a 37 percent clip.Essentially, a group of teams with an average pre-game Elo rating of 1478 played like they collectively deserved a 1419 rating. If the Eagles followed the same formula, we might surmise their “true” Elo rating without Foles would drop similarly to 1525, effectively making them the 18th-best team in football instead of the eighth-best.Making matters worse for Philadelphia, the numbers say that Foles is better than the typical starting quarterback — and that Sanchez is far worse. Since his NFL debut in 2012, Foles has been the sixth-most efficient passer in the league, according to adjusted net yards per attempt (ANY/A). Meanwhile, over the years in which Sanchez was the New York Jets’ starting quarterback (2009 to 2012), he posted the worst ANY/A in the NFL among QBs with 800 or more attempts.But when we look at the aforementioned dataset of backup-quarterback games since 1978, the post-game shift to a team’s Elo rating doesn’t bear any relationship to the disparity between the career ANY/A rates of the starter and the backup. In other words, the Eagles’ Elo rating is likely to decline with each successive game in which they are forced to start Sanchez instead of Foles, but it’s hard to say how much the difference between Foles’ career resume and Sanchez’s resume matters.Besides, there’s also evidence that Foles’s passing isn’t even the reason the Eagles have been winning this season. The team’s defense and special teams have been among the best in the NFL, while the team’s offense has underwhelmed. Although Foles’s numbers were otherworldly in 2013, his passing efficiency has been average at best in 2014 thus far, in no small part because the incredibly low interception rate he posted in 2013 has (predictably) regressed to the mean.There’s no question that the Eagles are worse off because of the injury to Foles, and it makes a material difference to their playoff odds going forward. But things may not be quite as dire for Philadelphia as they seem.