People have a hard time separating the output of a team or a unit (statistically) from the job someone does coaching it. I always reference John Wooden when this comes up, but you hear similar things from just about every coach who's posed the question: his best jobs coaching happened on teams that weren't necessarily his best teams.
This unit was well coached. Poor personnel--the worst personnel I can remember on a Cowboys defense going back to the Dave Campo regime--but they played hard, tackled well, and the backups and the replacement players came in, took significant snaps, and knew they're responsibilities. They got better over the course of the game, and over the course of the season. To a man, the players and other coaches speak glowingly of the guy. And he's mentioned in every conversations I"ve heard --literally--as a candidate for assistant coach of the year. Yes, I think they guys an outstanding coach, and I say that without even a hint of hyperbole. I think losing him is almost certainly a step back, unless we get incredibly lucky with his replacement.
What's more, I think that would be consensus in the coverage of his departure across the league...anywhere other than in the minority opinions of some Cowboys boards. We'll see, I guess, if it actually happens.