NFL's most improved players

Robert Quinn has had a great start to the season for the Rams. AP Photo/Tom Gannam

Since the best indicator of a player's ability is how well he performs on the field, when that performance drops, we look for excuses. Is it a bad system fit? Does the guy have an injury? Or is he just not as talented as we thought when he was drafted? And, because a number of things can explain subpar performance, the talent question can have us writing off players too early.

Many players can turn it around. Whether they've recovered from an injury, had something click within a scheme or just pushed themselves that little extra bit to be the best they can be, we can see massive shifts. Every year, there are guys who bounce back from disappointing performances and are vastly improved.

Using the PFF grading system, here are the five most improved players since last season.

Robert Quinn, DE, St. Louis Rams: +39.5
Remarkably, Quinn finished 2012 with our sixth-lowest grade of all 4-3 defensive ends. It wasn't that he didn't flash talent (look at his three-sack Week 5 outing versus the Arizona Cardinals), just that he couldn't string it together consistently. This year, he has done a better job in that regard and now leads our defensive end rankings. It's quite the turnaround, and nowhere is it more evident than in how much pressure he is getting. Last year, Quinn managed 45 quarterback disruptions on the entire season. This year, he's already up to 38, and we wouldn't put it past him to double his 2012 totals. It might just be that still-young Quinn is reaching up to his great potential. He came into the league in 2011 after a full season without playing.