It must be great for him to go from OKC, where he was the oldest of the young nucleus to Boston, where he is now the youngest of the core.
Instead of having to be a leader of sorts in OKC, he gets to be a student here. He was at best the third option most of the time in OKC, now he will be the first option with the second unit (and he will expose every opposing second-teams weaknesses - did you see how quickly Phoenix had to start doubling him?). He will even get good looks with the starters because of how unselfish they are. Rondo, Ray, Paul and KG are all willing passers and don't force shots.
The talent is obviously there, but his ceiling in OKC was already reached. A change of scenery was going to help him regardless, but I don't think any team could help him more than Boston.
Sitting next to KG every game will help this kid tremendously. I don't know how they did it in OKC, but it certainly wasn't KG's style.