Rich Rodriguez is the head coach. He hired Greg Robinson and thus bears responsibility for his performance. And he doesn't get a pass for one bad hire because he only hired Robinson after firing Shafer. And let's not even talk about Hopson.
So unless you're going to argue that it's not the coaches' fault that the defense is as bad as it is (and that it's not their fault that it's regressing as the season goes on...), then it's on Rodriguez.