Years ago, the league was a weight and power league. If you lined up bigger and more powerful players than the other guys, especially in the DL and OL, you simply dominated the other team and carried the day. Running the ball was the primary, safest way to advance the ball and the passing game was secondary in importance. In the past several years, the rules have changed and the league has become a passing league. Speed became the byword, not only on the outside, at the skill positions, but even in the trenches. There certainly are some large and wide bodies combating each other at the line of scrimmage, but even there, athleticism and speed has become more important. Take a look at the Giants DEs: Tuck, Ueminyora, Kiwanuka, Tollefson and looking back a few years, Strahan, were all speed guys, not power rushers. Furthermore, the LBs have become somewhat marginalized in importance in the league. A better way to say it is that the LBs were once the center of the defense. They had to be stout against the run and be quick enough to take decent drops against the pass, but TEs were not deep threats and they did not have to cover as often on pass plays. Now, most teams have speedy, pass catching TEs; teams throw passes often off of quick 3-step drops which calls for more quickness and speed from the defense, especially the LBs; and offenses using 4 and even 5 WR personnel groupings require defenses to use 5 and 6 DBs often, calling for LBs to be off the field. Complex blitz packages also require speedy LBs who can get to the QB quickly. My point is that in the previous era, weight and power were the things, now speed is the main determinant for winning.
Giants definitely bought into the speed thing. As I said above, the DEs are all quick and athletic; last year's starting S, James Butler was switched out for Phillips largely because of speed; last year's starting LB, Blackburn was supposed to lose his starting job to speedy FA LB Boley; Canty was supposed to replace an aging and slowing Robbins at DT and Rocky Bernard was supposed to help in the middle as a speedy and stout DT. These key additions to the defense all of whom were supposed to add speed have all been out virtually the entire year. Regardless of that fact that the Giants defense has some good players, they are missing the speed upgrade they thought they were getting and needed from the roster changes. Add to this the fact that Clark and Pierce at LB seem to be a step slower and the Giants have been transformed into a defense that is both smallish AND slow, a definite recipe for disaster. Add to that the troubles of the first year DC and the predictability and transparency of the schemes; the "communications problems", the missed assignments and the poor tackling (all coaching problems, BTW) and you can see why the Giants defense has been so poor against quality offensive teams.
Giants went for small and quick as the makeup of their defense. With the 2 fastest guys in two key units on defense (LB Boley and S Phillips) missing and the absence of Canty requiring too many snaps for the slowing Robbins, the Giants went from being smallish and quick to smallish and slow. The new DC Sheridan seems to favor a simple 2-deep zone, which puts more pressure on fast guys in the DB-field to cover the center. It's not working.
Having brought all this gloom and doom, let me bring one positive glimmer to the table. I watched the Eagles game again and the big plays given up by the defense were more blown assignments than they were physical domination by the Eagles. On the pass to DeSean Jackson right before the half, C.C. Brown was out of position. Corey Webster made some hand motion right before the play, banging his fists together, no doubt changing the defense and requiring Brown to cover deep on the right side of the defense. Brown did not get the message because he was in the middle of the field, a few yards away from where the other S, Michael Johnson was, who was patrolling the left deep zone. The result of the blown coverage was that jackson was embarrassingly wide open.
On the TD run by the rookie McCoy, it looked to me like the Giants DL lined up wrong; they were unbalanced with 5 or 6 guys on the left side of the defense and 2 on the right side. That's exactly where McCoy ran and he was nearly untouched for the score.
My point is that coaching mistakes and communications problems are at least theoretically fixable. Having bad players is not fixable. Maybe the Giants can get their act together and play better. We still need to see something from our coaches, that give us confidence that this can happen, however.