Millsap's the kind of 4 they need up front, and he makes sense if they can get him for 3 years max. They are missing his type of player to go with Horford. They have proven chemistry, and the Celtics supporting cast is much better than Atlanta's was.
I don't know who is going to overpay Millsap to go beyond 3 years. Not at his age. Anything can happen here.
If they miss out on Hayward, they could also go the trade route for a Butler or a George if extendable.