https://articles.masslive.com/celtics/index.ssf/2017/07/boston_celtics_summer_league_2_15.amp
Certainly the adjustment curve is a steep one for him, and he shows ample evidence that he's a worker.
Also, despite the reaction of fans, he has shown some assets in his brief SL career - he's relentless, he can catch a pass, he can finish through contact, he's coachable, he even seems to have good handles for a center.
But fans should temper their optimism, and my reasons were already evident in his Euro career. Why?
1) His body.
a) He has long legs. Not a problem once he gets moving in a straight line; when he eventually gets a feel for when he's got to change ends he'll be known as a big who runs the floor - the breakdowns in this area in SL are something that he can and will likely minimize in the future. The limitation comes in when he's got to move laterally, or close out - anything, really, where he has change locations quickly; it just takes something to move all that leg.
b) He has stiff hips and shoulders. To some extent this can be overcome with focused, regular work. But he's got a LONG way to go, and when you add that to the leg length issues it's hard to see him playing a big role in the coming season.
c) He has an extremely flexor-dominated body. Again this is something that steady, focused work can impact - but this will be a really slow process. This bias affects his ability to get off the floor and to accelerate - and again exacerbates the issue of leg-length.
2) He is a black hole. He has almost zero history of making plays for other guys. He certainly has made some attempts in SL, mostly with cringe-worthy results. That Boston has had an offense with Danny and Brad that runs stuff through bigs argues against his getting meaningful minutes next year (but see below).
I've said in the past that he was an NBA player and could be a regular rotation player - but not on the Celtics.
Things have changed though, haven't they? There are roster and even rotation spots for bigs that are ready and waiting for whatever low-salary guys can step forward and hold their own on an NBA court. In my opinion, Zizic playing rotation minutes in the coming season is likely to be a disaster for the Celtics - but given the opportunities available, I would not now rule it out.