My deduction from watching him over the years is that Rondo was indeed an elite defensive player in past seasons - up until maybe two or three seasons ago, back when the actual big three (Pierce, Ray, Kevin) were still together and were still significant.
In those days Rondo seemed capable of shutting down almost any PG he was matched up with. He used to often apply full court pressure (not unlike Avery Bradley has a tendancy to) and I would go so far as to has he was probably the best defensive PG in the league at that time...if not then I would cetainly say one of the best.
Last season his defense was pretty terrible, but I don't hold that against him because it's natural for a player to be a bit slower off the mark and a bit more cautious after going through a major surgery. The thing that concerns me though is that in the previous season BEFORE he got the ACL injury, he already had looked like his defense had fallen off in a major way.
I do have a theory for this. In the earlier Celtics years Rondo was really coming in to his own, but the team still belonged to Pierce, Ray and Kevin. Those guys had the ball in their hands a lot, and Rondo didn't really need to do so much on the court because each of those three guys was capable of hitting that big last minute shot when there didn't seem like there was anywhere for the offense to go. Likewise Pierce was a very good playmaker at the SF spot and was often quite capable of playing Point-Forward in spots.
These days Rondo has no real go-to scorers on the team, so everything in the offense basically has to go through him. He has to bring the ball up the court, he needs to run all the plays, and he rarely gets taken out of the game for a breather. Personally, I think he is probably just so exhausted from having so much offensive realiability, that he simply doesn't have the energy to play defense at the same high level he used to do 3 or so years ago. I think that's why he'll tend to sag off a lot more, pressure the ball a lot less, and often gets caught cheating on plays.
Rondo has all the tools to be an elite defensive player and I think he is very capable of being one today, but before that can happen I think the Celtics need to get him some support - another ball handler or two who can occasionally bring the ball up the court or run some plays so that Rondo doesn't have to push himself so hard. This is why I'd love to get a guy like Melo on the team, because he can create his own offense. From time to time if he's tired Rondo could just defer to Melo and he'd be able to take control of the game.
This is also part of the reason why I'd be pretty happy if Boston can get either Exum or Smart in the draft. Either of those guys is capable of creating offense either as a scorer or as a playmaker, so having either of those guys on the court alongside Rondo would I think help a lot to reduce some of that load.
