I think it's crazy to criticize Crennel too much. He lost matthieu and clowney, plus he hasn't really been allowed to acquire much in the way of high draft picks or free agents. Just has to find old fashioned value players.
Putting Lonnie on the outside, Roby inside, putting back mercilus into situations to get sacks and cause turnovers. Getting the big guys up the middle and big linebackers who can cover for his scheme. Has developed and acquired solid safeties.
Unlike obrien he understands what guys strengths are and where they'll succeed. He doesn't let a washed guy like Seantrel or mallet stay on the roster, much less win the starting job twice. He doesn't carousel, or sign a million guys at one position hoping one will hit.
That being said I don't really think there is enough talent on these defense to be consistent. There's only so much scheming you can do with the talent level. He doesn't have high picks all over the defense like the offense has, it's gonna be a grind. Lonnie and Joseph will get lit up and the pass rush will be intermittent