For the life of me I don't understand how people who watched the Celtics last year, especially in the playoffs, are concerned that Doncic is too slow to defend NBA PGs, and that since he's a PG, it's a problem. IT is 5'9" and can't defend NBA PGs regularly, so they threw him on taller, low usage wings, whom he gave up 8-12 inches to. Doncic is a foot taller -- the Celtics can throw him on those low usage wings the very same way, except they wouldn't have a height disadvantage.
And even if you think I'm wrong about the above, and that he will be forced to guard those 6'2" PG, logically that means that they'll have to guard him. And that's going to work completely to his advantage, as he'll have no trouble getting his shot off nor seeing over them to make whatever pass he wants.
I personally don't think the athleticism/quickness thing is actually set in stone -- he's just 18 and those attributes are probably still catching up to his size a fair bit. But he's got elite skills and an elite ability to see the court on both ends. He can be a star with only average athleticism.