I think you have to blame the coaching staff and Favre's starting streak as much as anything for his late season drop off.
Any other QB in the league would be benched if he were injured. With Favre, it seems that nobody wants to bench him. Nobody wants to be the guy who goes down in history as the person or persons who benched the QB with the longest starting streak in the history of the NFL.
It seems kind of stupid in so many ways. It would have benefitted the team and Favre if he were to have been benched towards the end of the season to rest his arm, and yet.......... it didnt happen.

While Favre should take a portion of the blame for this, I myself have to question the intelligence of the people in charge. I'm guessing this may have been part of the reason that Man-genius got fired.