Younger buy-low candidates and second or third tier free agents.
Greg Monroe will be the #1 guy in free agency, but when he signs in New York or Dallas, the Celts will look to some less-sexy options. They'll consider Brook Lopez, but he'll sign an insane deal with the Nets, or perhaps he'll go to Dallas or Portland.
The Celts will be involved in rumors for any interesting players that become available in rumored trade talks, e.g. Demar Derozan and Jonas Valanciunas, or Ty Lawson. Ultimately, I doubt they'll make a move, since nobody who becomes available will be worth it.
Next up, serviceable centers like R. Lopez, Asik, and Koufos will merit some consideration.
Then on down the list to guys like Bismack Biyombo, Wilson Chandler, Ed Davis. Perhaps trading for somebody like Lance Stephenson or Danilo Gallinari.
I wouldn't be surprised if they considered taking Jordan Hill off the Lakers' hands, or maybe absorbing David Lee with their cap space.