From my observsations:
His athleticism + some muscle he will soon add almost naturaly (remember he is still only 19) will put him among the superstar type players. That alone does allow him to dominate all the missmatches on the court easily. Even at this time he has shown that on drives and in a couple of postup situations allready.
His Offence:On offence he is very versatile. When Green was a rook pretty much all he could do is dunk the ball. JB can do that, but he is also very very skilled in the post, he can hit open 3s and draw foules. If he improofes his shooting every year like AB did he can become an All Star easily. Unlike AB he doesnt have to overcome limitations on Offence that come along with beeing untersized. He is a willing and allready pretty good passer.
On Defense:
His D is pretty good allready. He is good in one-on-one situations and has shown that he can protect the rim with his ability to jump and his quickness. He still has to learn not to jump after every shot and not to fall for every shot fake because thats what ppl will try to do to him as a rookie. The recent Knicks and Nets games have shown that. Ppl went at him a lot in those games. Carmello went at him as the rookie and gave him a free lesson. In the last game he also looked lost in the D at times. One was able to see the Imapct it made on him in his face. If he is really as smart as people say he will have learned a lot of things and will know that he has to work hard to even get a climpse on the level Smart, AB and JC have reached on that side of the ball, plus he has those guys to learn from.
Summary:
Jahlen has the physical tools to become a superstar type player, he just has to stay healthy. He is very raw still but he has shown glimpses of what he might become in allmost every game so far. People say he is a quickl learner and hard worker so provided his shooting comes around there is no reason for him not to become a superstar. He was drafted in a high motor team, a team with ambitions to at least progress further than round 1 of the playoffs, a team with a quallity starting 5 and possible the deepest bench in the NBA. The last 2 years have teached those guys that even with AH they still lack a superstar like DC but they also know that as a team they were able to overperform 2 seasons in a row. They know that in order to do that they need each and every guy on the team so they really work hard togather. They have recognized that JB allready can help the team on the court and that he is making good decisions. Given that he will not only get playing time, he will have quality vets to learn from who actually have a intrest to make him better and he also will have a good amount of competion for minutes. He might not become ROTY but he will most likely be at the All-Star weekend as one of the best of that rookie class. If he somehow is able to dupplicate the dramatic improofment of Rozier he could very well become 2nd year player of the year next season and give Crowder a real competion for his starter spot. Chances for JB to become another Jeff Green are very slim imo - he allways seems to be attacking the rim when he has the chance to.
AB is praising him too in this recent article on:
http://www.nba.com/celtics/news/sidebar/prac-101816-veterans-say-brown-perfect-piece-for-celticsHe allready made some waves around the NBA. People talk about him and media asks his teammates about him. They all have seen those glimpses and have given praise therefore my observations can't be too far off. Lets hope he stays healthy and does develop quickly into the next big thing.