This is quite a convoluted rabbit hole, imo but I think you might be thinking of best on best principles, unless defensive assignments made it back after Madden 2009 and was taken out again, which is possible. Apparently defensive assignments were first removed for Madden 2009 though and at some point I seem to recall someone pointing out it was removed because it didn't function correctly, which I know from experience was the case. You could assign a defender to cover someone then if that person wasn't on the field for whatever reason, the defender might not cover anyone, lol.
EDIT:
03-02 DB/WR Interaction by Ian Cummings, the addition of defensive assignments, improvements in route and coverage animations, jostling, and improved press play http://www.operationsports.com/forum...fo-thread.html
Well true to Ian's word, defensive assignments did make it back in M10 and that's where it went wonky and eventually removed again at some point. I'd have to find that too but it's at that point where I recall it being removed due to coding, that I referred to earlier.