This is just the beginning if I had to guess.
Really, they need to clean house of a bunch of guys. The defensive line and linebackers coach and special teams coordinator have been subpar for many years.
Part of this is injuries too, that's not an excuse just a reason. You can't go around missing your #1 corner, nickel corner, starting strong safety, 2 starting LB's and expect to be good as you would be otherwise. Still, when guys like Patrick Robinson and Corey White go other places and play really well you know the coaching is the issue.
IMO Ryan's defense in 2013 was good because he came in rushed and didn't have time to implement all his schemes. They were forced to play basic, man-to-man and rush the passer defense out of necessity and it worked. Ever since then they've gone to this passive zone nonsense that basically gives the other team free yards because you're playing 10-15 yards off the ball most of the time. It's insane.