Celtics fans tend to vastly overstate the impact Hayward has on the court. The Jazz let him walk for literally nothing and saw a whopping three win difference in their season record as a result. The Celtics lost him in the first game of the season and still made it to game seven of the eastern conference finals.
Hayward is a nice guy, but as the 4th highest paid player in the NBA is wildly overpaid and happens to play the same position as the Celtics two best young players. He's probably the player management would like to trade most.