During the Jazz game I couldn't help myself but feel that Castle has a significant drop-off when he's the only one of the three playmakers on the floor. When he's playing with Fox or Harper he knows there's someone else who can get the team going and doesn't really force things as much. But when he's the only playmaker on the floor, it gets rough.
While Castle obviously already has great passing vision and executes most of his ideas well, his ball security isn't on the level of natural point guards and out of all the things young player can develop, I'm a believer that natural point guards are born and Castle just doesn't look like one to me.
That isn't necessarily a bad thing because he already has legit forward size at 21 and is already an elite rim attacker with his relentless pressure, it's just that I think he'll be way closer to Jimmy Butler than Jrue Holiday trajectory.
On the other hand, Harper is obviously the best natural point guard on the team, regardless of being just a rookie. Whenever he gets the ball, he shows it and honestly games where he's parked in the corner shouldn't happen anymore. He needs to be the primary ballhandler whenever he's on the floor. Both Fox and Castle have proven they can excel in the secondary ballhandler role.
Fox has 2.26 assist/turnover ratio with 12.9 TOV%. (estimated turnovers per 100 possessions)
Castle is at 1.84 ratio with 19.6 TOV%. (yikes)
Harper is at 2.63 ratio with 12.2 TOV%.
Obviously Harper's sample size is lower and he mostly plays against benches, but he's already had enough games of elite passing and very few turnovers.
He's had just 2 bad games in that regard, in Orlando and in Vegas against OKC, everything else was honestly amazing for a 19 year old rookie.