I am not sure what to make of the defense. Rationalizing the poor performance, you could point to all the injuries that have compromised their personnel. They got virtually nothing out of the top end of the draft because their number 1 pick has played all of 2 games and their highly touted 2nd round pick is out for the season. They lost their top CB in Thomas, their starting MLB in Goff and several other good substitute DBs that weakened the personnel. They then had to go out and re-sign players that would probably not be in football otherwise: Grant, Coe and Martin among others that were already dropped and were forced into giving some of them significant playing time. Even there, some of these substitutes got injured and Giants were additionally limited on defense.
In addition to that, they built a defense that was totally predicated on big DL production and with Tuck and Ueminyora slowed by injury, they are not getting dominance from their DL. The one LB that was playing well, Boley is now also out and that really opens up some holes in the defense.
But that is on the rationalization front - the fact is that the Giants have drafted some very talented LBs, fast and athletic, and they have not done a good enough job coaching them so that this late in the season - they are still making weak rookie mistakes. By contrast, the Eagles came into the season overhyped but with real weaknesses at LB and S. Their players are not great, but they have improved to the point where they can play well and they certainly played well against the Giants last week. The Eagles defensive coaching also has some weaknesses, but they did get these LBs more ready to play and I don't see as much improvement from the Giants young LBs. Every play they make is purely because of athletic skill and not because of coaching and positioning.
You could look at it two ways: it's hard to judge the quality of the coaching because of all the injuries and patchwork lineup. For example, Giants are stuck playing nickel defense virtually all the time now that Boley is out, using Deon Grant on the field because they don't want to play the rookie LBs. That approach would seem to absolve the coaches. However you could look at it another way and say that the players can't be judged because the coaching has been so bad. It's probably a little of both - the coaches had a rough go because of the injuries, but they opted to go for a very conservative predictable approach and did not coach the players well to get them ready for the game, which clearly falls on their side of the ledger. Coaches saw weaknesses in pass coverage at LB and in the DB-field, so they elected to play very conservative, not blitz at all and load up the backfield with more pass coverage. Not smart strategy - when you are great, you can play conservative, but when you are weak is exactly the time to gamble and roll the dice.
I will say this - now that Osi is out, I expect the defense to get a little better. JPP will then be shifted back to his natural DE position where he can be more productive. JPP made some mistakes on contain last night, but is still better against the run than Osi and Osi has not exactly been lighting it up with his pass rush lately. . Kiwanuka should be moved to DE and the Giants should live or die with their young LBs. I'd rather have Kiwanuka as a good DE than a mediocre LB. You don't win by being not-awful at all positions. Rather, you win by being superior at several positions. Take a risk on the young LBs. Giants may not have a great defense with them at LB, but they certainly will improve with Kiwanuka at DE.