I would definitely move Hayward (and or Horford) for Davis this summer.
The coming months Hayward has the time to prove he's a vital part of the team or otherwise Ainge should be looking to move him in the above deal.
If that would happen then Stevens just has to accept it. Ainge always does what he thinks is best for the franchise even if that means making hard decisions that have severe consequences for players. He shouldn't let the coach get in between, because one of his favorites is in the mix.
That would be a personal decision instead of a personnel decision.