The defense was very good last year, and Gregorak is a very good and upcoming DC. That's why Stitt kept him and has said that he's basically turned the defense over to Gregorak and the defensive coaches. Stitt could have hired someone else, so his hiring of Gregorak says a lot.
UM had the top defense in the Big Sky conference last year. And the Big Sky had more offenses putting up big numbers than any other conference. The Big Sky offenses influence defensive stats.
UM was very good in things like sacks and red zone defense last year. The offense was better last year, and didn't end up putting as much pressure on the defense as it had in prior years, but there were still games like NDSU and others (even the EWU games) in which the offense didn't perform well enough to keep the defense off the field. It is also true that the defense didn't make enough stops on 3d downs.
Anyone who says the defense wasn't good last year, is either trolling or does not have a clue about the game of football. The defense and Gregorak were very good last year. They were far from perfect, and yes, they were not a "championship" defense, but they were good and often damn good most of the time.