To add to your point, I think NBA teams are more intrigued these days with players who show athleticism and defense in college because they often can work on the shooting, especially if it's a mechanics issue. You can't teach athleticism, and being able to play good defense at college shows good basketball IQ and a moldable player.
For an undersized guard (and Ball would be undersized in the NBA), they need to see quickness and a knack to disrupt plays. Steals is going to be a key stat moving forward. Diarra had 2x the steals Ball had this year, and he was doing that on one leg. That's the biggest area of opportunity for Ball on the defensive end, and he absolutely has to be better there.