Maybe we need to stop thinking of JJ as a quick PF and start thinking of him as a slow SF. He doesn't rebound, he doesn't contest shots. His quickness allows him to stay with most bigs defensively, so there's that. But what can he do once they post him up, or when they bully past him for an easy putback? Or when they make a quick pass to a cutting guard and he can't contest the layup?
He switches off well, and yet he's too slow to guard smaller wings, and he doesn't hit enough shots.
Basically he is giving us nothing right now. It is too bad, because he has nice speed for a man of his height. But the lack of rebounding or rim protection is a critical flaw in a guy who contributes so little offensively.