Fullback (Jerome Felton): The Bills went out and paid for one of the NFL's best fullbacks to block for McCoy; he'll be a big part of Greg Roman's offense.
Wide receiver (Sammy Watkins): His rookie season was good, but he can do a lot better if he can stay healthy and get some consistency from his quarterback.
Left tackle (Cordy Glenn): Former second-round pick enters a contract year and still must prove he's the blindside protector worthy of a big extension.
Left guard (Richie Incognito): It has been smooth sailing so far, but training camp will reveal if Incognito's season away from the game had any negative effects on his play.
Center (Eric Wood): Team captain is entering the prime of his career and the meat of his contract extension and needs to anchor an offensive line that struggled last season.
Right guard (John Miller): Third-round rookie drew immediate praise from coaches this spring but still has a lot to prove through the preseason to secure this job.
Right tackle (Cyrus Kouandjio): Like Miller, the new coaching staff is high on Kouandjio but he is still a raw prospect, barely having seen the field last season as a rookie.
Defensive end (Mario Williams): Rex Ryan's defense could use Williams closer to the middle of the line this season, making him more of a factor in run defense than as an edge rusher; he'll make his mark either way.
Defensive end (Jerry Hughes): Hughes was a part-time player in Mike Pettine's version of Ryan's defense two seasons ago, but Ryan appears to be making room for Hughes as the team's primary edge-rusher.
Defensive tackle (Marcell Dareus): The Pro Bowler should become one of the NFL's highest-paid defensive players when the Bills give him a huge extension between now and next spring.
Defensive tackle (Kyle Williams): Team captain is a rock in the middle of the line and has only gotten better as he's aged.
Outside linebacker (Manny Lawson): Unlike Pettine in 2013 and Jim Schwartz last season, the Bills could use Lawson, Hughes and Mario Williams on the field at the same time; Lawson will have to play in space as a coverage linebacker.
Middle linebacker (Preston Brown): Last year's third-round pick played the second-most snaps of any defensive rookie last season but will have to transition to anchoring a new defensive system.
Outside linebacker (Nigel Bradham): He enters a contract year but is back in the spot where he played in 2013; he played better in Schwartz's scheme last season.
Safety (Aaron Williams): Moving him to safety two seasons ago paid off, but the Bills need continued growth and maturity from Williams, who is an inconsistent tackler.
Punter (Colton Schmidt): He turned around the Bills' punting game last season after two shaky seasons from Shawn Powell and Brian Moorman.
Long-snapper (Garrison Sanborn): He's rock solid and one of the best in the game.
Punt returner (McKelvin): Thigpen performed well after McKelvin's season-ending ankle injury last November, but McKelvin was back in this spot during the spring.