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.
I think we'll still play with one specialist (primarily Carrick) and two all round box-to-box types. Now we have three very rounded midfielders, so that gives us more options to change focus during games to suit any scenario. In fact, I'd go as far as saying Schweinsteiger is, or at least has been, the most complete midfielder in the world.
If we can somehow capture Ramos, we'd have such a well rounded spine to our team, with Rooney being so rounded and versatile too.
On OP topic though, I'd rate Herrera up there with the likes of Ramsey, Wilshere, Henderson, Fabregas, Cazorla and Toure possibly as the most 'complete' in the league. Each have slight flaws and greater strengths, but they're all well rounded players. Herrera needs to show us consistency now to prove that he deserves a place among them, but I have to say, he does have a Scholes look about him with thoses short passes and quick movements. I can't wait to see him with our new additions next season.