I was at the game last night and there were definitely some eye-popping moments. That first block on AD nearly brought the house down. In his post-game interview, too, people were cheering for him. You can tell Celtics fans really like what they see from this kid.
Watching the game you can also see that he's very eager to get the block, which can eat you alive in today's NBA. Hassan Whiteside is a great shot blocker, but there are plenty of better defenders out there. Williams will need to learn how to do everything -- body up, be in the right position in the pick and roll, switch onto guards, rotate to the right man, etc. -- if he's going to be a complete defender.
For those clamoring for him to get more minutes... With Yabusele on the shelf for a while with that ankle injury, you'll get your wish. Horford has a knee that needs some care, meaning that he'll have to be thrown in there regardless of what you think of him.
Lastly: Have people seen video of the Celtics working on his three-point stroke? His shooting motion is decent and his FT% in college was fine. Three years from now he could be really valuable as a guy who catches lobs in the pick and roll, plays disruptive defense, handles the ball a little at the top of the key, and even hits the occasional three.