Heard Joerger on 1140 and he gave a pretty candid answer to Buddy going under screens when he shouldn't go under screens (like the Warriors' game). He gave a general answer to the specific question and said that the only real leverage he has is the bench; but when you're in a situation where the guy you want to "instruct" is a lot better than the guy that you substitute for him, that guy just "sticks his tongue out at you" on the bench and says, "See, that's what you get when you sit me down." Joerger says that as the roster gets filled out over time there will be competition at every position and that kind of thing won't be happening.
Lets look at the difference between going under, and through a screen. Are there times when it's beneficial to go under a screen? I would argue yes, there is. If the other teams PG isn't a very good outside shooter, but is quick, then it's beneficial to go under the screen which keeps him out of the lane and forces him to either shoot from out there, or pass the ball. But if that PG is Lillard, or Curry etc, then you have to fight through the screen or they'll kill you from out there with the long ball.
Against a PG like Rubio from Utah, we should have gone under the screen and forced him to shoot from out there, but instead we were trying to fight our way through the screen and allowing Rubio to get into the lane. So what happens then defensively? Willie leaves his man (Gobert) to move up in the lane and stop the ball. No one on the Kings rotates over to the basket, and Gobert scores on an alley oop. The first mistake was fighting through the screen. We should have gone under and forced a terrible shooter like Rubio to shoot from out there. The 2nd mistake was lack of recognition by our PF who failed to rotate over.
However, recognition comes with experience, and neither Bagley, in this case, or Giles has any experience. Of course the third option would have been for Willie to stay with Gobert, and make Rubio shoot a midrange shot. But, he is a much better midrange to basket scorer. Of course you were referring to the Hield situation, and obviously he was making big mistakes. There is a reason that the P&R is so popular. Because it works