1) It is an illusion to think that we have had poor defenses in the Belichick era. Only 2000 (17th), 2002 (17th), 2005 (17th) and 2011 (15th) have been out of the top 10 in points allowed. Even in these three years, the defense was about average in 32 team league. It can be argued that having an average defense is enough for success with a Tom Brady as the QB.
2) I suggest that we overestimate the 2001 defense (6th). Obviously it was a good defense.
3) 2003 (1st), 2004 (2nd), 2006 (2nd) and 2007 (4th) were by far our best defenses. It is true that 2007 is a bit hard to judge. HOWEVER, I would argue that I would expect MORE points allowed by a team that is always ahead. One might consider this year's DEN game, which wasn't a great stat success for the defense.
4) We are left with 2008-2013. All were in the top ten but 2011 (15th). We rank best in 2009 (5th).
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So, where does our current #13 defense in points allowed rank among the top defenses we've had? Should we only consider the defense of the last 6 weeks? That seems unreasonable. IMHO, we have a top 10 defense at the moment. We might have a top 5 defense for the playoffs with Jones back. However, it will difficult to judge, other than by a SB win. We could finish in top 5 for points allowed, or not.
BOTTOM LINE
Perhaps we have the best defense since 2007. Perhaps not. As I've said, I think that we've under-rated recent defenses. But if our defense is a top 5 defense, it would be statistically as good as any since 2007 (tied at 5th with 2009).
The defense SEEMS better because of the awesome secondary. Even with an adequate run defense, we need fear no one in this pass-happy league.