Umm... you do realize that he has been Manure's manager since 1986, right?
Marcelo Lippi has won much more per season in a shorter period of time. In ten years with Juventus, he won more on average than Ferguson. Plus, the trophy of all trophies, the World Cup, is something that the Manure manager doesn't have. Until he wins that honor, I don't think I can say he's greater than Lippi.
And jeez, I don't even think Lippi is the greatest manager of all time. It's just that Ferguson won a lot during the time when United were head and shoulders above the rest in England, a period of time which spanned for over a decade until the emergence of Arsenal and then Chelsea. Contrast this to what Lippi had to face in the 90's (Milan) and then the Roman emergence after the turn of the century. We all know Serie A was the far more difficult league from 1990 until about 2003.