I'm sure Steven's has his reasons for not allowing the kid to see such little playing time and all I can go by is the eye test.
But a few things are apparent. Besides being a "human basket eraser" near the rim, he moves so well, with little effort. He's able to challenge out to the 3 point line and then get back to his defensive assignment with ease. Many rim protectors can't do that because they're slow.
He functions well with the offense, he's not a turn over waiting to happen.
He gives the Celtics a wrinkle they don't have, and he's a hell of a lot of fun to watch.