UM runs a 4-3 defense with a lot of man coverage. In any 4-3 alignment, one of the most important elements is having playmakers at defensive end who can really get after the quarterback. That's their primary responsibility in the 4-3. UM's best since these coaches got here was Olivier Vernon and he was suspended for half the year and never really seemed to care too much. The lack of a quality pass rush, which UM didn't have last season, will really hurt any 4-3 defense. Having a completely non-existent one, like UM does right now, will kill the chances of success. It forces you to blitz a lot more...which leads to another important part -- having quality cornerbacks.
Brandon McGee has been average his entire career here. The other starter last year, Mike Williams, was a kid who couldn't even start at Wake Forest as a senior before coming here. Thomas Finnie, one of this year's starters, is more of a nickel who was forced into the starting rotation. The next 2 guys are true freshmen.
He has a playmaker at mike linebacker and will for a few more years. What he doesn't have, which are very important to the success in a 4-3 defense, is quality pass rushers and quality cornerbacks. Lets be honest, the roster at those two spots when these coaches got the job was terrible when you factor in Vernon's suspension/etc.
They added a pass rushing DE (Rico Williams) and a CB (Finnie) in the final 6 weeks of the 2011 class and then they added 4 D-Ends and 6 cornerbacks in the 2012 class. Some of them have bright futures here.
I think it's only fair to give him time. When his defense is filled with an experienced Tyriq McCord, Jelani Hamilton, Tracy Howard, Antonio Crawford, Denzel Perryman, etc then I think we'll have a fair evaluation of where this program (at least on defense) is headed in the years ahead.
Until then, UM will likely give up a lot more yards and points than the fans want to see. Shayon Green likely won't become an outstanding pass rusher overnight. McGee likely won't become a great cover guy overnight, capable of taking away the opponent's X receiver every week. It's going to take time.
A better evaluation until then, in my opinion, will be how well his teams tackle, how well they compete, how much they improve, and how much they can buy into his system and his beliefs. If all those things seem to grade well at the end of the season, it should bode well for the future.
Until then, you can simply be patient and trust that it'll work out over time.