The best time to have dealt Rondo was a year ago, but he was injured. Once you get to a player's last year, they hold most of the cards. All they have to do is say they won't re-sign with a team to scare a trade partner off. Even a team willing to take a chance will not offer full value due to the risk involved. Look at all the stars the past few years and count how many actually re-signed with their team before free agency.
I think Rondo, like any player, would like to make as much as he can while still playing in a good environment. One thing that may bother him is the feeling his last contract was below market value. This next deal will be his last chance to make big bucks and I wouldn't blame him for taking it. Remember, he wasn't even a high draft pick so his first deal was only 5.6 million for 4 years. Now he's been playing on this deal averaging 11 million/year for the last 5 years, while someone from the same draft class as him like Rudy Gay is making much more.
The final two interrelated things hurting the Celts' position is 1) they don't have anyone more important than Rondo on the team right now and 2) the team isn't very good. It's a lot easier to ask Rondo to take less if you've got two other superstars on the team who also need to get paid. The Celts' position is weak because you're asking the unquestioned best player on the team now to take a discount after he already took one on his last deal. What are you going to say at the negotiating table? That if Rondo took a discount again, it MIGHT make signing some theoretical stars more likely in the future? How is he supposed to react to that? Typically, teams that are good find players are more willing to take a discount while teams that are bad are the ones that have to overpay.
Jackie Mac made a good point recently that everybody is saying Minnesota has to trade Love or they'll risk losing him for nothing, forgetting that the Celtics are in the exact same position with Rondo. And we didn't even go .500 last season like the T'Wolves did in a much harder conference.
The best chance for the C's to keep Rondo if they aren't able to deal for a star before next year is if the market decides paying a PG who can't shoot 20 million/year is not a good idea. PG is a loaded position right now with plenty of capable guys who are making, or are projected to make in that 8-12 million range. I'm not saying Rondo isn't better than those guys, but is he 8 million better? 10 million? Can you win a title if Rondo is taking up almost 1/3 of your cap? Hey, stars like Duncan and LeBron are taking discounts now and that means if you're one of those teams whose star players are not taking less you're actually at a double disadvantage, because 1) your max guy is not as good as their max guy and 2) their max guy is now actually taking less money.