The problem is two-fold: pass rush and coverage at safety.
On pass rush, I'm quite sure both Alexander and Hughes are hurt even though they are not on injury report. Alexander had to miss games recently and Hughes went out and came back into games due to injuries in recent games. I haven't seen effective inside move from Hughes for awhile now.
After a few games, opposing teams got a handle on the kid Lawson and he could not tying up two blockers to free up the rushers behind him yet. He needs to learn the art fast.
Kyle Williams has played at a steady level, surprisingly healthy thus far. Just imagine where the defense would be if he hasn't been there on the line.
The hope is really on Dareus. He's really just getting started so he hasn't lost focus like he was in the past. Rex must be praying that he stays healthy and is available the final 5 games.
On coverage, they probably peaked in the 4th game against NE. Ever since Aaron Williams went down, they started going south. I guess no one listened to Ed Reed other than Williams because communications in the back suffered the most. You can see the Ryan brothers tried everything they have, even sacrificing the contain on QB just to provide help.
They have to do something there. I don't know they can go outside to get help at this stage, but I'd put White back at safety, even if Darby can't go. Push Seymour to the outside, let him swim or sink - we are sinking on all of those safety help routes anyway, might as well try a different way.