it is difficult to judge jj since he did not play a whole bunch. but when he did play i thought he had a nice mid-range jump shot, was athletic, and fairly quick.
the downside i saw was that he seemed to have no grasp of the defensive schemes doc uses. i mean that he was lost time and time again on switches. he was obviously used to using his athleticism for defense, but that was not enough for the celtics defensive scheme.
next problem, he is thin and weak by nba standards. he was pushed around under the basket by most other pg and c. since he needs to make a living playing pf, this is not good. it also means he can not fight for rebounds or box out effectively.
but...perhaps, he might be able to put on pounds and muscle, so perhaps this is not a fatal flaw.
next, next problem... i did not see an ability to pass or make other team members better. maybe this is due to him being a rookie, but when i saw him in limited minutes, he simply did not appear to be the type of player who passes and makes others score. this may seem minor - maybe it is - but given his other flaws it seems to me that he has limited skills. i dont see what he brings to the celtics aside from a nice jump shot.
at his best, he may be an active, scoring pf who can boost the bench. but even then, i fear he will forever be a defensively limited player who can not rebound well or lift the team to greater heights.
in short, if jj can be a nice role player and scorer off the bench for the celtics, i would be happy.