Available now:
Dallas is an obvious seller right now. Davis is probably too risky and costly, but Gafford should be available at a reasonable cost. He's more of a traditional big, so I'm not sure if that's something the Cs are interested in at roughly 16.5m/year for four seasons including this one.
Available after this season:
OKC after this season will have a lot of tax pressure, so guys like Hartenstein, Dort, and Caruso might be available. Hartenstein will be making 28.5m on a team option OKC will likely pick up, but an extension would probably be less than that per year. He would be a much better fit in terms of switchability and toughness/rebounding, but he's not an amazing rim protector nor does he spread the floor. There will probably be a lot off demand for him, however, due to his toughness, championship experience, and versatility. Maybe if Boston is one of the options Hartenstein wants to go to, OKC would do him a favor since Presti is a Mass. guy and has a good relationship with the Celtics (he helped Horford get back to Boston, remember).
Cleveland has a humongous tax bill this season (163m-ish according to Spotrac), so regardless if they win the title, they will likely be trying to unload some contracts. That means Jarrett Allen might become available. He's kind of a traditional C so it's not clear the Cs would be that interested in him at an expensive $30m/year. Also, Cleveland would probably prefer not to deal him to a rival.
Some other possibilities:
If Giannis decides he wants out, Myles Turner might become available.
As some have mentioned, Zubac might be available if the Clippers continue to stink.
Not good fits:
Vucevic is likely available, but he's too old and horrible on defense.
Sabonis is probably available soon if not now, but he's horrible on defense and makes a lot for what he actually does.
Porzingis has been a little disappointing in Atlanta and it's not certain they will definitely re-sign him. However, he's extremely risky due to his POTS and other injury concerns. I could only see it if nobody wants to give him a big deal and he gives Boston a discount on a short term deal to rebuild his value.
Out of all the names, I would think the Celtics would be interested in Hartenstein first and Zubac second. I can see them not really wanting Gafford since he might make too much if you think he's better as a backup on a contending team.
Turner is obviously intriguing with his stretch 5 and blocking capabilities, but I wonder if the Celtics really like him considering Turner was in trade rumors for years but never went anywhere when he was in Indy.
