Other than having NO RIGHT HAND at all, I think JY's problem is he's got a case of the yips!! I'm sure just shooting around he probably makes 18/20 threes. In Red Claws games, 10-12/20. No pressure at all! In C's games he's lucky if he makes 2-3/20. A "shooter" who can't shoot because of nerves!
It's not the kind of thing that you can overcome playing on a contending team with sporadic playing time. Practice doesn't help, DLeague games don't help. Only being on the court, playing lots of minutes, and feeling like "you belong" in the NBA will do it. He'll never get that in Boston. He'll be released/traded first. It's too bad!