I think two of the biggest things impacting our defense is field position, and penalties.
Right now our team is giving up the 3rd worst field position... our opponents start on the 30 yard line... compare that to the Bengals, whose opponents start on the 21. If the opposing team drives 30 yards (a fairly average drive), they are getting into deep FG position (i think? 57 yrd fg?) against us, while the Bengals have forced a punt. That adds up over the course of a season.
The other, penalties... while we're middle of the pack in yards per game on the defensive side, we're in the top 3rd when it comes to first downs due to defensive penalties per game... 2.7, vs a team like the Bengals, at 1.38. Again, this sort of thing adds up.
We're giving up an extra first down, and giving our opponents 10 yd shorter field, compared to other teams in the league. This makes it much easier for our opponents to get scores.
We're going to need better play out of our offense, and importantly, better play from our special teams, to help our defense keep our opponents in check in terms of scoring. The defense isn't great, but definitely not bad... it seems to be top 3rd to top half in most categories that aren't impacted by the above two statistics.