IMO because of his end of the game scoring aggressiveness, Turner will be missed.
But the signing of Horford is a piece the C's really needed. He's a real nice fit, allowing the team to be more effective going big and small. He's in his prime, he's a good interior defender, something the C's need badly, he scores well inside and outside, he runs the floor well, and is a real solid team guy. The team simply got better on defense, and offense with him coming aboard.
As said previously, Brown is the X factor... will he be a rookie that needs time, or will he be able to successfully attack the rim, develop an outside shot, and defend like he's known to. IMO If he's able to reliably contribute immediately, then the Celtics could end up in the eastern conference finals against the Cavs.
Green, is an ok pickup, I just don't think he takes the place of Turner at the end of the game.