No doubt, Bob Diaco had some good years here as a defensive coordinator. Then again, he did have quite an impressive stockpile of defensive talent to work with.
If he were really that good of a defensive coordinator, I would have to assume that someone would have wanted to retain him as a DC after he had been fired from his job at Connecticut.
He has had one year stints a Nebraska, Oklahoma, Louisiana Tech, and Purdue, in some form or another, but none of them wanted to retain him as a DC for more than a year. That, right away, should say something about his true potential.