It's true that the job was too big for him too soon, but then why appoint him in the first place? Fergie has a lot to answer for in how instrumental he was in his appointment, and both he and Woodward with the garbage that was left over. The team needed players at the start of Fergie's final season and Fergie knew it. By the next summer there were holes everywhere. Now Fergie has basically shat on Moyes saying that the job was too big for him.
Just so happened that Moyes got it wrong trying to get United playing like an Everton Max, which was never going to work. Fellaini flopping just compounded matters. Mata was an example of the quality they needed, but he came too little too late. It's almost like they knew there would be a settling in period, but they thought they could keep their CL spot and the money ticking over. It would be very interesting to see United rattling around 6th-9th around Feb-Mar. But they'll most likely sign a strong DC in January and they have enough quality to beat most teams at home.