If the Celtics could trade Rondo for Deron Williams over the summer, I'd be on board. But Williams is an unrestricted free agent who can't be traded until he signs with somebody. Which means he'd have to decide he wants to come to Boston as a free agent. In that case, the Celtics would be able to shop Rondo - why shop him just to New Jersey? New Jersey would be losing Williams anyway, so they'd actually have to give up something of value for Rondo, like Marshon Brooks, or a sign and trade of Lopez.
There would be some salary advantages to dealing with NJ (the Celtics would only have to free up about $6-7 million in cap space in a straight sign and trade of Williams for Rondo so they might be able to keep both KG and Green) but wouldn't you be better served signing Williams with cap space and trading Rondo in a sign and trade for Hibbert, for example? (Rondo would absolutely get you Josh Smith or Al Jefferson, he has way more value than those guys). Probably still have the space to keep Green under those circumstances, or KG if he'd take something below, say, $8 million (not sure he would). But a starting five of Williams, Bradley, Pierce, KG and Hibbert would be a hell of a beginning to a team.
But in the end, it's still about whether Deron Williams wants to go to Boston, which means he holds all the cards. If he'd rather go to Dallas, he'll do that.
The other thing to consider in this is that if you're looking at rebuilding, Rondo will cost about $6-8 million less per year than Williams over the next three seasons. So it's gonna come down to the details.