To be fair to Moyes, Ferguson did a poor job of building for the future. He gambled that players like Cleverley, Jones, Smalling etc would not only be the core of the England side for the next 10 years but that they would be top players as well. As it's turned out they're no where near good enough to replace the old guard of Neville, Scholes, Giggs and Beckham.
The signing of RVP was also incredibly selfish. United were only signing young players and had previously ruled out signing 'old' 30+ year olds. However, Ferguson was absolutely desperate to go out on top and threw all the money at him for one last hurrah, hoping he could get one season out of him being fit. The gamble paid off for Ferguson but now Moyes is having to put up with all the injuries.
Then you have the likes of Ferdinand, Vidic, Evra & Giggs who are all getting on and need replacing as well. The cost of replacing these older players, upgrading the crap English players and bringing in a new striker is gonna be absolutely huge.
All this isn't even including the whole Rooney affair that Ferguson left behind. For me, Ferguson failed to set United up for after his departure. He is mostly to blame for everything which is happening right now. I'm sure a world class manager would have United higher up but they'd still need to so a huge rebuilding job and they still wouldn't be near the title.