The Celtics have never won anything without a guy to plug the paint and bang the boards. That's not Horford's strength. They knew that when they signed him, but the coach is in love with perimeter type bigs. Guys who can shoot the 3 and pick n'roll and defend it.
Noel is an inside defensive guy, an Everett kid who wants to play in Boston. The Celtics are maybe putting the squeeze on Philly, still hoping the Kings will trade Cousins, or just plain don't want another big.
I said before I heard Danny say personnel decisions are made equally by Wyc, himself and Stevens. Judging by Stevens' taste in bigs (or lack of it), I would say 50/50 they do nothing.
At present, they are little more than a .500 team, so Danny will have to take the reins sooner or later.