Even though there is some team continuity, in the NFL every season a coach gets a new team, it's never the team he had the year before.
In New England, Belichick takes that team from training camp & builds up as the season progresses. The best time to play NE is the 1st month of the season because Belichick is still working the bugs out, but as the weeks go on NE just keeps getting better & better, building to peak as the season gets to it's latter stages. That's why the Bills could beat the Pats last year & Arizona could this year in September. Even back in 2003, the Bills were able to win 31-0 on opening day, but in the last game the score was reversed. Most injuries don't matter because by the time the season is ending Belichick has either coached up the new backups to be able to come right in and play, or cut the deadwood & replaced them with a veteran who can step right in.
In Buffalo we get worse as the season goes on. Injuries always pile up & every year by the end of the season we get the excuses that next year all the injured players will be back & we can't possibly have that many injuries because this year is an obvious aberration. Yet every year by this time, we have guys in there starting who won't be in the league next year. We're not seeing rookies on the rise getting plugged in in December. Well coached teams get better the longer the season gets. Poorly coached teams get worse & get injured.
Also, we need a good strength & conditioning program, something we haven't had since the Super Bowl years. Ever notice how just about all the key players were in there for the playoffs during the Bills Super Bowl years? Yes, Kelly and others missed a game or 2 but virtually none of them were ever out for the season in January.