My 2 cents:
Mata is currently way better an alternative to Herrera at No.10 role.
That leaves the CM spot for him to fight for as first eleven. Those that just look at the midfield and scratch their head at why Herrera is not starting are not looking at the whole picture. The reason why Herrera is not starting is NOT because of Fellaini Carrick Schweinsteiger or Schneiderlin. He is not starting because of Blind. With Blind at centre back, you lose a position that is required for aerial domination, therefore you will need to compensate by sacrificing another position with the likes of Fellaini. It was so apparent against Southampton and that is why we lost instead of scratching out a bore draw. This is what they call a domino effect.
If we do get a centre back like Vidic to pair up with Smalling, then Herrera will have no problems slotting into the CM role with Carrick, Schweinsteiger or Schneiderlin.
It is painfully obvious. And it is painfully frustrating that we have no solutions to this. The only solution is the transfer market, where we either get a commanding centre back whom can also play ball, or get a Yaya Toure type player (in his prime).