The thread title says "three years later" - what?! He was drafted two years ago.
As far as a verdict goes, I'd say that he's an NBA player, with upside as a rotation player. He has an unusual build/skillset, which is an advantage. I don't think that his body will last at this level into his early 30's, maybe less.
As far as his future with the Celtics goes, I'd predict that he at least finishes the year. They think that he can make decisions, initiate offense, and shoot with range.
I just see a lot of scoring talent. Just with his 3 point shooting and ability to attack closeouts, I could see him getting buckets. Combine that with offensive rebounds and post touches.
I'll take these in order.
We could quibble about what "a lot of scoring talent" amounts to; I wouldn't say it that way. He does have a couple of ways to score. Big hands and great feet, good handles for his size, has some ability to drive it. If you believe the tiny sample size, he finished really well.
In spot minutes before the All-Star break, he looked like a reliable 3-pt shooter (.364) and an exceptional offensive rebounder (12.6%). After the break he averaged low rotation minutes and those numbers came down to earth. Because of the tougher defense playing against rotation players? Random drift? Rookie wall? All of the above?
Post touches? I'm skeptical. More likely is the new wrinkle that we saw in Summer League: they used him at the top of the circle as a facilitator, and with some success, too. He got a couple of nice assists out of it... That looks like a good role for him, initiating the offense à la Olynyk.
You did not mention his production at the foul line. He got to the line a lot for the possessions he used; he was a C-minus free throw shooter for the year, though A-minus after the break.
I fully recognize that we haven't seen the production for that, but I'm saying i've seen enough to think he has the talent for that.
I'd agree that he has the talent for it, and I'm with you when I say that at these sample sizes you've got to do some careful looking.
The sample sizes are tiny. There's a tendency on forums like this to put too much weight on FG%s and 3-pt %s, and regardless of how many shots we're looking at. (That's not an argument that he's actually in truth some kind of great shooter, either!). My rule of thumb is, what does the % look like if you add or subtract two or three makes? There's enough random stuff that goes on during a game and during a season that you need to look at a range, and that gives you a better sense of what to expect in future.
For the year he shot 12/37 for .324; that's below average but it's also about a point per shot, which is acceptable production for a possession. If he made 9/37 that would be .243; if he made 15/37 that would be .405 - that's a huge range, so the first thing to take away from his actual number is that it does not give you a good idea of his shooting. He's somewhere between catastrophic and golden.
Great footwork and size should translate into better individual defense and especially defensive rebounding. The defensive rebounding is critical for his future, and he's got a ways to go - with his butt, he really ought to be better at it. I thought he looked clueless on D in Summer League 2016, and especially during early offense, so he's advanced a fair amount, I'd say. "Better but not good."