Ander won't be sold, the rumors makes very little sense and are reminiscent of last season when Van Gaal's words got micro-analysed (he's not 100% proficient in the language still), and Herrera was being linked with moves away from United, prior to him complying with the all-encompassing philosophy. We finally have a midfielder that can add some vibrant link-up play and goals from central areas, and his effervescence will provide a huge spark going forward, despite additions of even Der Fußballgott and Schneiderlin to the likes of Carrick and Blind. All of them are more holding players, and presently can't play the role Herrera can, who brings balance in terms of primarily being a central midfielder who can add goals and link with the forwards in the final third.
Fully expect to see this kind of base setup with minor changes to preserve fitness or account for injuries/ loss of form :
----Schneiderlin--------------------------Herrera---
------------------------Fußballgott-------------------
Schneiderlin and Fußballgott can both play in the #8 box to box role, but the latter has lost a bit of mobility, so I'm not sure he's ideally suited to that role. Some of his best performances for Bayern and Germany have come from a similarly deep position. Morgan can play either the defensive box to box or as the #6, did so for Southampton to good effect.
Carrick in another holding midfielder, and given his age will slowly be phased out with one eye on the future or maybe even start at first until Fußballgott is more accustomed to the league. Blind is the jack of all trades, adept at filling in as and when required, as either the #6, or as an emergency left-back. Not sure how Mata will fit into the midfield three, he's probably been penciled in for the right flank like the 2nd half of last season. A bit too lightweight for a Van Gaal midfield. Could also play when we mix up the formation a bit, necessitating the presence of a traditional #10 behind the striker. So that's all bases covered, not sure Van Gaal would like to disturb the balance now that we finally have a good complement of midfield players.