How do you figure the Bills have taken a step back on the defensive side of the ball? Who did they lose, outside of Greer, who missed 6 games last year and with the Bills adding more DB's, and Crowell, who didn't play at all? They'll get Schobel back, Ellison will get a full off-season and training camp as the starter (instead of being thrust into the role just days before the season starts), they have great depth now in the secondary, added a 1st round pass-rusher, and have a new DL coach.
Even on offense, again using my worst-case scenario, the Bills have Butler and Walker at their old positions, have Hangman at center, a promising rookie starting at LG, and Chambers or Bell at LT, with some help. They lost Peters at LT, and he surrendered 12 sacks while not giving a damn for most of the season. The addition of TO and Rhodes are huge and help the O-line out a lot.