People notice Jim is glum of late relative to the last couple of years. I'll propose it is because he is a smart enough to acknowledge over-abundant evidence that come year end he will face a Lloyd Carr fork in the road, having to choose between loyalty and extended mediocrity on offense, and making a change at OC.
Drevno and Harbaugh have worked together a long time. Quick research (maybe a mistake or two):
One can espouse that while Jim has lifted Michigan especially with the Brown hire, Drevno increasingly smells of Hoke, where there was magic early, but the wheels eventually came off.
There is still time to right the ship and we'll all be rooting for Tim to get the offense going. But at the current trajectory, the most important decision that may separate us from a decade of championships or one of mediocrity will come this off season.