The Giants were missing their 2 starting CB's I believe
Not that I have a clue on who
should be the Giants starting CBs, but according to a quick search, Banks and Flott (I have no idea who they are) were their projected starters according the "unofficial" depth chart the Giants released in early August:
Both of those guys played vs the Cowboys.
Now their projected starting Nickelback, Phillips, did not play. But the backup Nickelback Simmons did.
Again, I don't know who SHOULD be the Giants starting CBs, but if someone was missing, not a single CB on their roster looks to be a standout / make a big difference guy.
JMHO