First of all, coming from someone who has been following him since highschool, some of his weaknesses have been extremely overstated in the past couple months. People honestly think he is extremely raw and doesn't have a feel for the game, when that just isn't the case. I'm not saying he doesn't have a lot of work to do, because he does. But he also has the chance to be a complete player and if he reaches his potential he will be a star.
He has been a top5 recruit since high school and has been throughout the year until March came around. He had a rough last few games and people have reacted severely, literally off those last games. Also, I see a lot of people who think he can't shoot because he shot 30% form 3 this year. He shot 38% from 3 in AAU/National stage game in High school, he isn't as bad a shooter as people seem to think. Another knock has been his handles/passing. He got a lot of those TO's this year from either being careless with ball or from offensive fouls from using his strength(will be less penalized for this in nba). He also actually has exceptional vision, decision making is definitely still something he needs to improve on though. Some of his struggles also had to do with his team being horrible at spacing.
The way He fits in with the celtics is that he is the perfect versatile defender. He can guard 1-4 with ease. He is so strong and athletic that he is such a mismatch and he can cover essentially anyone on the floor. He's also extraordinarily intelligent, and that should definitely help him in the growing phase. It might take a few years for him to put everything together, but he could become so good especially in our system. Think of him and Jae in a small ball lineup at the 3 and 4, we would create huge mismatches. He has a bigger wingspan than Lebron! Oh and he also was Pac-12 Freshman of the year and made the Pac-12 all 1st team. He's a beast and everyone seems to be sleeping.