It's not that difficult to grasp. We have a squad with some decent players in it, and a manager who either misuses them or ignores them in order to use other players who for one reason or another simply aren't good enough.
Kagawa is a perfect example. He hasn't contributed enough, but I've no idea if he's good enough or not, because he rarely gets picked, and when he does, he's played on the wing. The usual mantra is to not trust him because Ashley Young is fit, which makes no sense for two reasons.
THe squad for me clearly isnt good enough in certain areas, but Moyes fails to get the best out of it or even use the best players effectively in areas that shouldn't be of concern.
Last night was just a perfect example. 7 of the players he picked shouldn't be near a United team based on their performances this season (Vidic, Rio, Evra, Cleverley, Carrick, Young and Valencia) and Smalling ISN'T a fullback. So 8 unstaisfactory areas, and at most 3 where you can argue he didn't have players available who would have done better (Evra, one of Vidic/Rio and arguably one of Carrick/Cleverley).
He should be selecting teams with one or two weak areas that he's unable to address. Instead he's picking teams that have so many holes there's barely a player left in it capable of doing his job.