JJJ is not a one on one stopper in the post, especially with FIBA rules making it ok for the offensive player to basically assault a defender. He’s not a great rebounder either. He is elite at guarding on the perimeter for his size and is elite at providing backside help defense.
It’s a roster construction issue. If you are gonna have JJJ, you have to have another big to do the dirty work with him (look at Steven Adams with the grizzlies).
And they should spend all pre-tourney camp working on taking the ball off the rim on shots. No Goaltending in FIBA after the first bounce. A ball should never hit the rim twice. Euro guys know that