I think over the next couple of years we'll be able to see his winning aura, his influence on referees (whether implicit or explicit) and his psychological grip on other managers in the league (whether as henchmen or bruised rivals) were worth about 5-10 points each season.
That's aside from the fact he was obviously a very smart and well-funded manager. He signed players who could play the way he wanted - aggressively, with an emphasis on width - and he reacted well tactically on match days.
So overall the drop to Moyes is probably 3 or 4 wins, maybe 5.
