First, they are not as bad as you may think. Going into this game they were fifth overall.....going by yards. After this game they will take a big hit. Sometimes the higher ranked defenses are on teams that have weak offenses....the games tend to be low scoring and close, kind of like the Bills of the last couple years kind of fall into that group. Things have changed, the offense can score points and put the other team in a hole. When you have a team that does that it provided a good chance for the other team to get a lot of yards and points. They need to learn how to play with a lead, get to the QB and cause some turnovers.
I think the D may just not be quite as good as we have thought of them as, they need to evolve this year to form a better match with the offense. They played a lot of weak teams last year, a lot of QB's with very little experience. This year the schedule is loaded with quality quarterbacks, we say that yesterday. McVeigh and Goff have had a lot of success, they were a super bowl team two years ago. Moving forward, its Carr, TanneyHill (Not Mariotta), Mahomes, Darnold, Newton, Wilson, Murray, Herbert, Garrappolo, Roethlisburger (not the chumps from last year), Lock, Newton, Fitz.
Overall that list is way superior than the list of nobodys they played last year. Maybe there will be some changes, but all in all, this year is a lot more challenging for this years defense.