While it's true that Fergie is the best club manager in Europe, and has been so for at least a decade), what Fergie has to work with is a squad second only to Chelsea. They may be weaker than his squads of the past, but they're still a more cohesive unit with a lot of very good players, which shows more how incredibly good they used to be than how poor they are now.
2 seasons ago (when we finished 2nd to them) it read something like this: Players in LFC squad worth >18m - 5. Players in Man U squad worth >18m. 17. Since then they have sold only Ronaldo.
*That* is what Fergie has to work with. You won't find United finishing 7th any time soon, with or without Fergie, though maybe without him they won't win the title quite so often. Having the right manager is important, but it's far from the only thing.