I would argue that Moss was never Brady's most valuable weapon - that would be Wes Welker. Moss got the highlight reel plays but Welker is ALWAYS open and picks up a consistent 10-15 yards a play, generally on 3rd down as well. Without Welker, Brady would get a lot more coverage sacks and wouldn't look half as good as he does.
The fact that the Pats offense didn't even skip a beat when Moss left shows both how adaptable a QB Brady is and how much of a non-factor Moss is in the offense.