The problem here is that fixing the defense and the midfield was not some monolithic issue which should've taken special priority over retooling the entire squad. Improving the attack while tightening the defense can and has been done simultaneously in the past, and United should have been no different - it's hardly a befuddling conundrum. Also, did we really fix the defense and midfield in earnest? De Gea was one of the top keepers in the world before Van Gaal took charge, Shaw was Shaw, Smalling (this extends to Jones too) just needed to stay fit and find some consistency instead of playing makeshift rightback. A lot of the defensive problems were predicated on the performance of a couple key players - and those problems have resolved themselves due to favourable circumstances. Take one of those key players out - De Gea for Romero, or Smalling for McNair, and it will fall apart. As for the midfield, we still aren't playing someone who IMO is our best midfielder - that would be Ander, because he doesn't 'fit' into a double pivot which we're insisting with for some reason. Schweinsteiger as good as he is - is ageing and it will show from time to time. We stumbled upon a very effective midfield trio last season, with a good balance between work-rate, distribution, creative guile and verve. And for some reason - completely abandoned that, and went back to the drawing board during the pre-season, instead of building upon the good work of 2014.
For the attack - he has to take full responsibility. Whether the personnel is deficient, or the system sucks, or everyone bar Rooney doesn't 'get' the philosophy. Letting Chicharito, Van Persie, Welbeck, Kagawa, Falcao, Di Maria go within the space of 12 months, aside from loaning Januzaj, and buying just Memphis and Martial was always going to backfire. Everyone could see this coming from a mile away save for Van Gaal, or whoever drew up the plan to jettison half a dozen attackers while pinning our hopes on kids from substantially inferior leagues - who were always going to be inconsistent given their age and relative inexperience deserves 99% of the blame. There's too much unnecessary experimentation and tinkering instead of laying concrete foundations and adhering to a strict blueprint, which intuitively goes against the argument that he's laying the platform for future United managers.