Umm, he left him a squad that won the league and had confidence coursing through its veins. Maybe they achieved what they did because of the 'Fergie effect' but all that means is that one of the biggest clubs in the world had one of the best managers. And that is what they wanted from Moyes - someone to show true, top-level managerial ability.
Moyes deserves his share of blame but I also think Fergie got out at just the right time. I wouldn't call them lucky to win the title last year but they certainly were not the most talented in the league they were just very good at grnding out results and Van Persie was the difference in front of goal.
looking at the squad Moyes was left he is left with several young players who I would argue are not good enough for United (yet or ever in some cases) (Jones, Smalling, Welbeck, Cleverley, Zaha, Rafael, Fabio, Januzaj, Buttner, Evans) on top of for various reasons players who have went off the boil and are dead wood at their club (Young, Anderson, Fletcher, Nani, Hernandez, Kagawa) The final piece of course is Giggs and Ferdinand are basically shot and Scholes papered over the cracks the past few seasons for them.
I think their squad needs a complete overhaul. They obviously have some great pieces with Rooney, RVP, Vidic, Valencia, Evra, Carrick but the overall talent level especially in depth is nowhere near good enough for United. The Fellaini purchase was very bizarre he doesn't fit their style of play at all and that has proven to be a big mistake, however Mata should make a huge difference midfield creativity is exactly what they need so maybe Moyes is getting it