Guys who do it in football generally have years of college games behind them to establish their credentials and value, and the vast majority finish out the season anyway. In college basketball, it's almost unheard of, which is why you had to point to college football in the first place. Who was the last college freshman who did that in basketball?
Johnson also didn't just quit at some logical stopping point - he quit in the middle of the season, and after two really shitty games and reduced playing time. That was long after he'd already recovered from his preseason injury.
In top of that, he'd quit his IMG team the previous year without any explanation at all, and ended up only playing 9 games for a high school team. Which means that solely by choice, he's played in only 22 games total in the last 2 years. By his choice, and without any explanation that's worth a crap.
Sure, Coach K said the right things...because that's the kind of thing he'll always say in those situations because he's not going to say anything publicly to hurt a kid's chances.
22 games in 2 years, by choice, is a giant red flag to me. Whether he gets anxiety, or jdoesn't react well to competition or adversity, or just gets homesick, no thanks.
Until he proves otherwise at a level that matters, he's a mentally soft quitter who needs to show he can wear big boy pants consistently.