I think there is a need for one player to go at some point just to get the roster down to 15 but I suspect there will be more roster turnover than that before the season starts.
If it was me, I would be trying to move Evan Turner during the preseason. I think he is a good enough player to be of value to a contending team (which I do not consider the Celtics to be). He is just on the wrong point of his career arc to be of real value to the rebuilding Celtics. I loved the Otto Porter trade talk. I think there could be a trade there that could work for both teams.
Once the season starts, I would be looking to move David Lee. Like Turner, I think Lee will prove good enough to be a serviceable player off the bench for a good team but even more than Turner, just not a good fit for the Celtics in terms of career arc. Lee requires a lot of money coming back which makes a trade hard so we will see.
As for the rest of the team, just about everyone else could potentially be in play for the right trade