It's tough to rate defenses. Points scored are influenced by a host of factors including opponent field position. Yards given up is influenced by the performance of the team's offense.
I've devised a measure that looks at average yards per play assuming that that 56.7% of plays run are passes (the league average). By this measure, the Giants have the seventh best defense in the league in 2013 so far.
The Giants' defense ranked 6th in yards per attempt versus the pass and 5th versus the run. The funny thing is that they did this with only the 25th best pass rush in the league. They were the only team in the top 7 in defense to do this other than San Francisco.
Whatever went wrong in 2013, it's hard to blame it on Perry Fewell and the defense.
The defense is going to feel some heat. Sure, they played well for a duration, stopping many of the league's top rushers. But they didn't beat one winning team all year nor a top QB.
Jon Beason, Antrel Rolle, Prince Amukamara, Will Hill and the interior line all played well. Justin Tuck had a bit of a renaissance.
They didn't embarrass themselves but good offenses ate them up early on in the season. They were just as responsible for the 0-6 start as the other two units. They gave up long drives and big plays in those games.
I like the way they finished the season. The last ten games they were fairly solid. More is needed, however. Depth at corner and LB plus the pass rush needs to be relit.