Why not just have KG take JJ in for a month or two this summer and train him up? But do we have evidence that KG can train up a player?
As for Jabbar, well, I don't care who he played for, if he can teach...but can he teach?
You know some people can do, but they can't teach, some people can't do, but they can teach, and a very few people can both do and teach. The Celtics need someone to teach/coach the 4/5 positions. That someone needs to be good at training 4/5's, not necessarily someone good at playing 4/5.