Man U won't go into meltdown. Ferguson's achievements were down to the size and resources of the club. He could pretty much anyone, with the ability to meet any fee and any wage demand. He could afford a huge squad. He can offer CL football, and an array of sponsorship opportunities, private jets, and much more. None of that will change in the short run. It goes to demonstrate that Fergie is no genius, that his achievements are in direct proportion to his resources; in fact it could be argued that he under-achieved by his relatively poor showing in European competition.
But for Moyes, the challenge will be rising to the level that Man U as a club expects; its "entitlements". This is the honeymoon. The marriage is the hard part!