Thomas, Bradley, Smart, Turner, Sullinger, Olynyk, Young, Zeller, +2 rookies = 10 roster spots. Wallace they will keep through the fall in case they need his contract in trade. That leaves 4 spots.
I think they absolutely match offers on Crowder. They've got an affordable team option on Pressey and they've kept him around this long, so there's 2 more.
Then they go hunting big game. They'll offer the 2015-16 max or something close to it (which they know is a formula for a future bargain) to Leonard, Butler, Love, Jordan, Millsap, and Tyson Chandler. I think they'll get one. One of these and one lesser player is also remote possibility.
So there's one spot left, maybe none, before we even talk about Bass, Jerebko, and Datome.
It feels like it will be time for Brandon Bass to go and I'm guessing the feeling will be mutual. Bass is a remarkably level guy, but usually a player spends that long on the trade block they develop a desire to somewhere they feel wanted.
I doubt the Celtics will make any serious effort to retain Jerebko or Datome if someone else pursues them. If no one does, though, I wouldn't be surprised to see either get a roster spot here.