Kevin Porter Jr. was actually a good man to man defender at USC the issue was off ball. Thats where KPJ really needs to improve his game on both sides of the ball. He's a good with the ball on offense and a gritty on ball defender. But off ball on defense he lacks awareness and effort and off ball on offense he just stands around.
People said pretty much exactly the same thing about Sexton a year ago, and he proceeded to be one of the worst defensive players in the league. KPJ won't be quite as bad just thanks to his size, but the point is good on-ball defense alone doesn't get you very far in the NBA.