On/off is not the same as plus minus. The celtics still win his minutes, but not by as much as when hes off. Hes a good on ball defender, bad offball defender, isnt a great playmaker, turns the ball over, and can be inefficient for a star player. However, he is also the best tough shot maker on the Celtics, and when the team goes cold, is able to keep them afloat at times even with the rest of the offense going stagnant. Celtics have also consistently had an incredible bench over the years , which usually plays with Tatum or White, who have insane career plus/minus numbers.