don't doubt that someone who didn't play college football could coach a college defense. It might not be the norm, but it is possible. The biggest problem is that Howell doesn't have the respect of the players. Part of that may be that he didn't play himself, but I think that could be overcome if he had the right personality/coaching style to gain the respect of the players.
Once again, I'm not defending Howell personally. I'm defending the idea that you have to play college football to be a good coach. Theoretically, if someone was able to do this, they should do a lot more while working their way up to a bigger program by being successful in lesser roles.
What I'm saying, is if Howell is not a good coach, not playing is only a small part of it.