The Horford situation from last off-season should have taught us that anything is possible (no pun intended), but I still think it’s unlikely that Hayward doesn’t work something out with the Celtics, assuming there’s no hidden bad feelings between Hayward and the team that we don’t know about.
Even though we all thought Horford was interested in staying, in Hayward’s case you’re talking about a player who signed on to play with his college coach, and has written about what that meant to him in terms of making the decision.
Horford’s case actually should be a warning to Hayward. Don’t take your current situation for granted. You might sign on for more money somewhere else and end up in a much worse situation basketball wise.