The below list is for this year only:
The Untouchable (Group A):
Unless you're offering Lebron, don't bother asking
1. Horford - Not only is he either the best or 2nd best player on the team, it's a simple fact that you can't trade a top notch FA the same year they signed. It's not against the NBA rules but you slit your own throat with agents for years to come. It'd be one thing if the player was a major clubhouse problem but with a pro's pro like Horford there's simply no way to do it. If you trade Al this year then the Celtics are stuck building solely through the draft because no FA worth their salt would even sniff the Celtics. Thus his chance of being traded is zero. Nil. Zilch. Zip. Ainge won't wreck the reputation of the Cs across the league with every agent by even whispering of moving Horford.
The Almost Untouchables (Group B):
It'll take someone's left nut, their first born child and their undying gratitude to get these guys
2. IT - He's been the face of the franchise the last year. He was in the meetings with Al. He was at the draft. He's on the posters. He's a locker room leader. I think if there was a deal for a guy like Westbrook then you could see IT as a centerpiece going back but that's the only way he's leaving this year. While in theory you could bring in a replacement for the scoring and the distribution duties given to Smart, it would have to be in a truly special deal. Plus there's the issue of how IT's ability to drive really sets the offense in motion. No one else can do that. If you put Smart back there then you have defenses sagging off of him a bit. If you put in Rozier then they don't have to respect the passing as much. He's the straw that stirs the drink right now and it would be a major adjustment to this team if he wasn't running the show.
3. Crowder - It's not because Crowder is the next best player (he's not) or part of the heart and soul of this team (he is). It's because there's no one to replace him. Brown isn't going to be ready to step up and take his role this year and there's simply no one else who can play the 3 and small 4 roles. So unless the deal is for a SF - and that wouldn't make sense given Brown is the heir apparent - then Crowder ain't going anywhere. I'm sure he and his contract will be asked about quite a bit in trade talks but removing him from the team leaves a gaping hole that the Cs just can't really replace right now.
The Painful Partings (Group C):
This needs to be really, really good or I'm hanging up!
4-6. The next 3 are the guys it'd be painful to let go: Smart, Brown and Bradley. I like Smart's toughness and winning attitude. I like Brown's potential. I like Bradley's professionalism and continued growth. But to get something good the Cs are going to have to give up something good and thus it's likely that one or more of those guys goes if there's a big deal. Still, it'll take a good deal to get one of these guys. They're not going for average veteran help.
Everyone else (Group D):
Sure, I have a few minutes to talk
Everyone else falls into D.