People are focusing too much on the second unit and having guys come off the bench. The Celts don't need to move one of the starters to the bench, Brad has no trouble managing the rotations so that one of the starters is always on the floor. It is rare, outside of late game blowouts, for the Celts to have five bench players on the floor at the same time, there is usually a starter or two on the floor. There is simply no advantage to moving Hayward or anyone else to the bench since it is very easy to set up the rotation to keep a starter on the floor. People are talking as if the Celts bench is out there with no scorers, that is simply not happening.
It is not so much about the 2nd unit as it is about the starting unit.
About making sure everyone is getting enough touches and is being involved in the offense. That hasn't been happening early in the season with Jaylen Brown in particular being on the periphery of the action.
Moving one starter to the bench in favour of a low usage bench player (like Baynes, Theiss, Smart) gives the other 4 starters a better chance at starting off the game in an aggressive and effective way. And allows the starter who moves to the bench to do the same because he comes in to replace one of the others.
It is to get the maximum possible out of the best players.
From there, the question is who? I say Hayward over Brown or Tatum because his superior ball-handling, passing & shot-creation make him more useful as a go-to guy in less effective lineups than Tatum or Brown.