Brad,
Remember Pat Riley spoke about the disease of me. If you don't know about it...google it. That is what's going on with this team.
You finally have a ton of talent and now expectations are through the roof. Coaching marginal players to play good is great. Coaching good players to be great requires tough decisions, hurt feelings and brutal honesty. Coaching great players to be accountable and show leadership requires a sit down with those great players to lift others up. It's called sacrifice.
Hayward is a team guy. He has no problem sacrificing and going to the bench.
Horford is the engine that makes your scheme work, that's why you signed him. You don't want to trade two FAs in Horford or Hayward that came to Boston willingly...it will look bad after the IT situation. Players talk.
Kyrie is the assassin, he's the specialist when all else fails. He has to be the guy that may sacrifice the most to improve the team.
Sit these 3 guys down as the guys who have been deep in the playoffs and tell them they may collectively need to sacrifice to allow the young guys to grow and be confident. The vets time isn't now, it's in April and beyond. I believe you're dealing with factions among the team...its vets vs young guys. The young guys believe they can do it themselves because they went to the ECF. So randomly sit some one of these 3 periodically and let the young guys sink or swim. Once they get their confidence back you can mix things up more freely.
You're playing for May, not December. Play the long game but young players only see the next game because they have everyone in their head. The older players know what's down the road.
Good luck, I believe you can fix this. Talk to Ainge, Pierce, KG...they are all at your fingertips to help and talk to the team about suppressing egos.. Right now these guys aren't playing Celtics basketball...the calvary isn't coming, you are the only guy that can fix it.