It strikes me as very Van Gaal to play a very precise strategic model (and to try and get each and every player to really understand it and know it inside out) but to do so with players who are well rounded and not too specialist. So the team can be tactically rigid (philosophy philosophy philosophy) but situationally flexible: Herrera may be the attacking midfielder but if he gets isolated in a defensive situation for whatever reason he's not going to disgrace himself.
City under Pellegrini work as a contrasting example: ideally, they don't want the likes of Silva or Navas to have to do any defending. It's one of the reasons Touré so often just doesn't quite seem to work in their side, because he's not really what their system requires from a player in his position (where Vidal or Pogba might be a better fit). If Herrera was playing ahead of Toure instead of Silva, for example, he'd be much more likely to drop deep and play more box-to-box in order to compensate for Touré's lack of work rate if necessary. But Silva's not really meant to do that, so those inadequacies stand out.
Carrick doesn't qualify for all this, of course. He's very much a specialist in line with the trend over the last five or six years which you pointed out: an out-and-out quarterback like Busquets, Pirlo, Alonso etc who could easily have his best game without ever getting within thirty yards of goal.