He'll get more minutes after he improves his defense. Must still be learning the intricacies, there are honestly way too many for a rookie to pick up. He's better off working mainly in practice and film sessions with small opportunities to prove what he's learned. No reason to over-expose him to offenses he doesn't know how to defend properly. Although this method is slow, the Celtics are currently contending and need all the wins they can get, so they can't afford to experience rookie mistakes all the time.
Also, he needs to improve his ballhandling and composure; he seems a little nervous when the ball gets into his hands even though he occasionally can make good decisions.