Since Crenell left in '05, the Pats D hasn't been nearly as effective.
'06 AFC Title game against the Colts - blew a 21-3 late 1st half lead and ended up losing 35-31.
'07 Super Bowl - blow a 7-3 4th quarter lead against the Giants, including the game winning drive by them in the final 2 minutes. And for that matter too, they got exposed in the 2nd half of that season, even against mediocre offenses.
If anything, they've gotten progressively worse overall. It's no coincidence that BB added Capers to his coaching staff this year, so would think Pees would be on a shorter leash if the Pats D continues to regress.