I don't like to talk about the mental state of players very often because it's something we really have no way of knowing about but I just can't think of any other reason for Justin's struggles other than his lack of confidence.
He dominated a summer league game last year after half of the other starters went out with an injury. He got sent down to the G League at one point during the season and dominated a few games down there as well. On the flip side he has completely disappeared from games unlike any player we've had on the Kings since I can even remember. We've watched a lot of bad basketball but I can't remember a player that routinely put up as many near goose egg box score lines than JJ has.
I think it's a mental thing for this dude. He has the skills. Look at his highlight clip from last night and it's not just floaters and 3 point shots. He knocked down a turnaround fade away and also had a sweet pull up mid range jumper over a defender that looked exactly like the move Kevin Durant destroys teams with.
If he fails out of the league, I don't think it's going to have anything to do with his physical skills. I think it's going to all be in his head. I don't think anything else holds him back other than that.