Herrera is not a CM, but Blind can play there obviously so they're fine.
I think Schweinsteiger still has a lot to give. His first season has involved a lot of injury and terrible tactics, maybe under Mourinho he'd find a new leash. The Schweinsteiger Martinez combo was so good to watch.
I think there are more issues than a CM, a creative winger with pace, a striker and a CB would be prioritised more if I was a United fan. One injury away from playing Ashley Young or Januzaj is not a good thought.