Peter King's column addresses this issue. He spoke with a former NFL player who now studies depression in players. http://sportsillustrated.cnn.com/nfl/news/20130115/nfl-playoffs-peyton-manning-peter-king/?sct=uk_t12_a5
He wasn't nearly the player he was with New England. He had a couple good games and inspired the Bills to thwomp NE in the first game. After that, he was pretty average.