Weaker defenses are nothing new to Mike Shanahan.
In his 16 years as a head coach prior to this season, Mike Shanahan's offenses have ranked #9 on the average DVOA while his defenses have ranked #18. For all but four of those years, since 1999, he has had control of the entire team. So, the responsibility, both credit and blame, for those results ends with Mike.
He has obviously done much better with the offense than he has with the defense. Since the offense and defense are equally important, his defenses have been dragging down the quality of his teams over the past 16 years.
I was unimpressed by the hiring of Jim Haslett, but firing Haslett is unlikely to effect a major turnaround of Redskins' fortunes on the defensive side of the ball. He hasn't been Shanny's DC for 16 years.