I don't think that Manchester United needs Sanchez.
Already have some good players like Martial or Rashford, and Mata who can play as a right wing.
Perhaps, if Wenger et Arsenal accept this deal (Miki deserves to play more), it will be a nice thing, but not a necessary thing.
They have good players but I would pick Sanchez before Martial and Rashford any day :) He must accept the deal or else he lose Sanchez in summer anyways.