Well, Eli Manning is the best down-field passer in the NFL. I can't remember the site where i read it, but the statistics supported him being the best and most accurate down-field passer. Plaxico Burress, at 6-5 232 pounds is like a power forward, able to break past receivers who jam him at the line to go down field. His replacement is 6-2, 185. About a fifty pound and 3 inch difference.
Domenick Hixon may have good NFL-level skills as a receiver, but the way he plays may be much more different. He may go across the middle more, take longer to get downfield, or run faster in routes. The difference isn't just production, but how he affects the playcalling.
So Burress' absence may have affected the play-calling (because the receivers have different strengths) and the Giants might have to adjust blocking for different routes, seams, and different lengths of plays.
The effect is much greater because Burress is such a specialized player with unique talents. Its much different than a generic wide receiver. And his strength is also Eli Manning's strength (downfield).