We did NOT have one of the worst defenses in the league last year. We gave up 19.4 points per game, 10th in the league. The run defense was terrible, no doubt, but we made it up in the passing game, where we placed 7th. Admittedly, some of that is from teams not getting into passing situations. It wasn't a dominant defense by any stretch, and it certainly wasn't good *enough*, but it wasn't terrible and it wasn't one of the worst.
Add to that the fact that Whitner, Simpson, Youboty, Ellison, McCargo, and Williams are almost certain to improve after a full year in an pro system, Hargrove should pick it up as he gets used to being in Buffalo, and the team's defense will be pretty good.
In any case, it's the offense that causes problems. 3rd worst in the league in yardage, one of only six teams to average less than 100 yards per game on the ground, and abysmal on 3rd downs.