He'd be stupid to NOT take some extra time. He shouldn't let ignorant people, who have never torn their ACL, pressure him into coming back too early, re-injuring himself, and ruining his career.
You mean all those ignorant people like Dr Andrews the world renown orthopedic who did his operation and will clear him for play. Or all those other ignorant people like his physical therapist, the Celtics's team doctor and the Celtic trainer, all of which also have to clear him to play?
Those ignorant people!!!! How dare Rondo listen to them!!!! 
First off, even if your surgery is a success, there is still an extremely high re-injury rate. Secondly, 99% of athletes (yes, even pro athletes) develop imbalances from their recovery time, which can lead to them tearing their OTHER ACL very easily (which accounts for most of the re-injuries).
Listen to someone who has had multiple torn ACLs surgically repaired by surgeons who work on NBA players...or listen to a bunch of people who have never torn their ACL give their opinions that are based on nothing more than how quickly "player x" came back.
Rondo is going to listen to a world renown orthopedic surgeon, Dr. Andrews, who is the person that did the surgery and set in place his rehab program. If Dr Andrews clears him to play professional basketball, then Rondo should be fine playing professional basketball.
That is the person Rondo will listen too. If you are doubting Dr Andrews ability to gauge when Rondo will be ready, then you must either be a better orthopedic surgeon that has seen Rondo's medical records or, well, site rules prohibit me from saying what the other thing is you are, but let's just say, its not a complimentary term.
Also, as long as Rondo remains on a preventative care rehab program for keeping the ACL strong, re-injury is relatively low.