There's a laundry list of issues. Chris Jones has flashed Aaron Donald type potential, but he's been brutal the past few games. Justin Houston is being misused (covering Antonio Brown? wtf) and still doesn't seem completely healthy, or otherwise the injuries affected his explosion. He was always a big key to the defense. Dee Ford is probably one of the worst guys in history to have a 10-sack season; he'll usually have one speed rush per game where he just blows by a guy with speed, but is otherwise quite bad. Derrick Johnson is just OLD -- father time has really, really got to him. Then you've got the secondary which is just a disaster. I don't even wanna go there. Safe to say Eric Berry was a RIDICULOUS loss, though. It's one thing to be just 1 of the best defensive players in the league, but he was the absolute heart and soul and vocal leader -- apparently no one has stepped up in his place. They don't ever seem to make a big play after giving up one, which is what they used to do.
At this point you can probably fire the DC, too. I don't think he's to blame THAT much, but the scheme isn't working anymore. They used to "bend but not break," which was kinda fluky, and now they're just broken.