First link above from Bangee7 in the other thread. Both links describe in detail how bad his defenses were and why he was fired from his past two gigs.
Horrible move. It explains our worst fears -- we got the equivalent of Joe Barry - just a guy with a horrible track record of bad defenses. As has been said, it's certainly possible the good D candidates said no because of job security (ie. too much risk of Gruden getting fired in 2 years & having only 1-2 years in the role). But it's ludicrous to believe Manusky makes our D competitive. Best case scenario is Scott McC drafts well / gets good FA players, which automatically improve the defensive play despite Manusky / not because of him.