My attempt to solve the Marcus Smart mystery:
The advanced stats tell us that the offense is much better when Marcus Smart plays. The basic stats tell us he's a point guard that can't shoot and that Terry Rozier is a much better offensive player. So why is his impact on the team so massively positive? Simple: he's a true point guard and Rozier isn't. This is not a knock on Rozier who probably has greater value around the league. But what Rozier is right now is like a smaller, raw Russell Westbrook. Or maybe Marcus Banks with a jumper. Basically he can score and he can defend but right now he's much better at putting the ball in the hole than running the team. Marcus Smart can't seem to hit the jumpers that you would expect an NBA guard to hit but he is actually the best on the team when it comes to finding the open man (Irving could be better but when a guy is a scorer his mind is largely focused on scoring and less on the open man). So when Smart is out it's almost like the Celtics don't have a point guard...
...please don't take this as any kind of disrespect to Kyrie Irving, he is still team MVP by a wide margin. It's just that...this forum hasn't had a post in half a year. I'm kind of embarrassed.