While I agree that the assist numbers were quite low, his usage on offense was very specific to:
1) Spot up shooter in the corner - where he was the last guy to receive the ball and was expected to take the shot after it swung around to him.
2) Slasher - often getting the ball on the move and pulling up or getting to the rim. His ability to get fouls called (and sometimes make the and-1) was especially impressive for a rookie.
As others have said, once he tightens up the dribble, he should be fine and, next year, Brad may let him be a little more creative with the ball. He does seem to be emulating the AB role, which doesn't bode well for AB's future in Boston.
The guy's edge, poise, work ethic, and IQ are really great factors that should help accelerate his growth. There is a lot to be excited about with Jaylen Brown.