(probably more in the 10-15% range especially acquiring a similar package).
And here's the rub. They aren't getting a similar package for Rondo. I would be shocked if they get a likely lottery pick and a young star (remember, Gordon was very highly regarded before he was traded, and turned to glass) for Rondo. One of them? Maybe. But both? Very unlikely.
And THAT is why they shouldn't be trading Rondo. Not because of Rondo, but because they likely won't get enough value for him, particularly when you consider that he will be coming off a knee injury this year, and next year will be his walk year, killing his value.
I made this point right after he was hurt and there was talk of trading him.
The timing of the injury and contract plus the landscape of the NBA, there is no reason to believe the Celtics will get good value on Rondo.
Depending when a Rondo trade happens, he is:
a) Still injured, hope to have him back by the end of the season.
b) Just off the injury, hope he can regain his former self.
c) A one year rental who likely will be looking for a raise
All the time playing the currently deepest position in the NBA (completely aided by rule changes of the past few years)
So, no, Rondo does not "have" to be traded.
The Celtics "have" to be open to the idea in case some team decides to ignore the reasons and offers a great package for Rondo.
Rondo can not be "untouchable", only the illusion that he is.