Welcome to the 2013 version of Insider's offseason staple series: Red Flags. This is the first of an eight-part examination in which Football Outsiders identifies the biggest remaining issue for every NFL team, division by division, after the NFL draft.
The series begins Monday with Outsiders' look at the AFC East.
Buffalo Bills: Cornerback
The past three seasons, Buffalo's pass defense hasn't finished higher than 22nd according to Football Outsiders' DVOA metric of play-by-play efficiency (explained here), the Bills' pass rush hasn't ranked better than 24th in that span. The offseason spending spree that added Mario Williams and Mark Anderson before the 2012 season probably would have improved that ranking, if not for Anderson's 11-game absence. (Williams had 10.5 sacks, so he did his part.)
If Anderson returns to his double-digit sack form of 2011, the only thing standing in the way of an above-average Bills pass defense (finally!) is their underwhelming stable of cornerbacks. Aaron Williams, one of last year's starters, struggled so much that the new coaching staff decided to move him to his more natural position of safety for 2013. The new staff seems to have given Leodis McKelvin's career new life; after two years of his contributing in the return game and on nickel downs, Buffalo re-signed McKelvin to a four-year, $20 million contract and gave him Williams' old job.