I have in mind a roster that looks something like this:
-Rondo, Bradley, Green, Sullinger
-a center who can anchor the defense and protect the rim
-a sixth man type who can come off the bench and hit threes and provide credible defense against bigger SGs (I think Bradley can defend those guys, but he might need some protection against fouling out) or a starting SF who can push Green into that role
-a bench that could include Olynyk, Faverani, Crawford, Pressey, Bass or Humphries (but probably not both), Wallace, and Lee. Not all of those guys, but some of them.
Getting that center, combined with a motivated and healthy Rondo, plus some additional time for Brad Stevens to fine-tune his schemes, could have the Celtics as a top-five defense. I'd argue that if you do that, it wouldn't take more than an average offense to make the Celtics a team that won't be a favorite but has a realistic longshot chance at winning the Eastern Conference.
The Celtics are multiple players away from being a contender. Instead of acquiring all of those players at once or drafting them and waiting for them to develop, perhaps it makes sense to acquire them one at a time, maybe one during the season when some teams are selling off talent cheaply in an attempt to tank, another during the off-season, and possibly a finishing piece for bench depth during next season.
Obviously, I am thinking of Omer Asik as a possible step in this plan, although it doesn't have to be him.