it's not the defensive coordinator, I don't think. the problem is RR likes to fit square pegs into round holes until the sheer force of he insertion changes the hole from round to square.
this year for example, RR clearly didn't have the depth to run a Defensive Back-heavy scheme like the 3-3-5. he only had one healthy DB returning from last season, a bunch of freshman and walk-ons. talk about an empty cupboard, right? but instead of adjusting the scheme to maybe a Nickle or 4-3 or some such, he persists in the 3-3-5 and the defense got assraped.
flash back 2 years when RR gets to Michigan. He didn't have the personal to run the read-option spread with Threet at QB. So what does he do? Well he runs that offense anyway! The kids tried but embarrassed themselves because they just weren't those types of players.
RR is totally inflexible when it comes to having or not having the tools he needs. He will run the schemes he wants to run no matter what. If RR has the depth he needs at DB to run a 3-3-5 and be effective, maybe the defense can be fucking great. I'd much rather he come down from the mount and run a normal defense...but if RR needs a 3-3-5 to work, he'll have one more year to have some healthy and somewhat seasoned players who understand their roles in the defense better. That's all the time he has. But it's also what he should get.
People who don't like Michigan seem to quickly (and conveniently) forget that this year we were down 2 starting players in our defensive backfield, had to run a converted fullback at linebacker, and our best DB was...a walk-on. On a defense of all freshman and a few sophomores. That just collectively weren't talented enough and that truth didn't change as the season went along. Fans knew that going into the season and so did the haters. The question is how suspect will they be going into next season. RR's job is riding on it.