Answer: Rondo does not have to be traded.
Rondo "may" be traded at some point, but it would have to be a very good deal for the Celtics. Since this year is probably not a Championship year, they have plenty of time to get his rehab right, while still building toward a championship team...time to actually test out young talent.
Even then, if Rondo was traded for less than top talent, it would be a disservice to the team and to the fans.
You may think that during the rebuilding stage it will be fun to watch young talent develop, and it is, but it wears thin after a while. (It was fun for Cleveland watching Lebron develop...and then what happened?)
Rondo will not be an easy player to replace. Practically to a man at Media Day event, players stated that they looked forward to playing with Rondo, that he made other players on the team better. (Besides KG, who was the last Celtic that other players said that about? Larry Bird comes to mind...) It is a hard skill to find.