Going strickly by the question , "ONE PERSON" , the ONE PERSON that could be traded easiest and bring what the Celtics need the most a true quality "BIG " is Rondo.
Nobody else is worth a ONE for ONE trade to get what we need.
There are alot of guys I would trade trade BEFORE Rondo if multiple deals were considered, but going strickly by the OP's question. Rondo is the only guy that could be traded to bring a "BIG" in as an "IMPACT PLAYER"
This is a good question. And it might ultimately come down to DA to choose Rondo or build the team around a young quality center.
Whats the difference of building around a young center or a young PG? The league is not a center dominated league, actually right now I would say it is PG heavy. Either way you need more than one good player, the point is to add a player to the good player we already have.
Name the last time a team built around a PG won a title? (or went to the finals)
NBA teams in most cases have a top level big man to win a title.
Real exceptions - Jordon's Bulls, Detroit bad boys, and a case for Lebron can be made (though I don't think they get past the Celtics if Bosh doesn't return)
All of Phil's teams played the triangle and didn't need a PG at all. Or a big man, really. But they had the best rebounder ever in Rodman. He counts.
The Bad Boys had both: a HOF PG in Isaiah Thomas and a burly (jerk, dirty player, dirty person) in Laimbeer. Big Bill was an animal inside. He might be an actual animal. But it was enough to best the best frontcourt of all time two years in a row and win back to back titles.
But the "build around" concept is fundamentally flawed in an era where at least 2-3 stuperstars are necessary to get to the conference finals. If one of them is a PG, fine, but you still need another star or two.
"who to build around" tends to depend on which star you end up getting first. No team with one superstar is going anywhere these days (except to another team, apparently).