The more I think about this, the more I think it may happen.
Considering how difficult it is to recruit in Jan and how few targets are likely to be available (for a sensible fees at least), the planets may just align Utd going for Allen. He would fit Utd's style of play, he could play in either the Norwood or Lundstram roles which Utd need competition/cover for, by all accounts has a spot on attitude, I'm sure he would welcome a move back to the Prem, and Stoke would most probably welcome the opportunity to offload one of their biggest earners, all of which may make Utd keen (again?) and Stoke willing to deal.