Rondo will get offered a max deal, which will be roughly 4 years, $82 mil (depending on the very uncertain salary cap) by multiple teams. If Danny wants to keep Rondo he should offer that. If he doesn't want to, he should trade him today. As Ainge knows that Rondo will get offered this and hasn't yet traded him, my expectation is that he plans to match, and offer a 5th year player option to trump other offers.
Would I offer that? He's my favorite Celtic, so I may be biased, but yes. But if I wouldn't offer it, again, I'd trade him ASAP.