I expect JJJ to contribute some this season. Where an AB style jump is very unlikely I do expect him to be a contributor on night where KG gets rest.
I honestly think the lack of camp / summer league hurt him a lot. Where a player like Moore is use to playing against other athletes who are bigger and stronger then him JJJ needs some time to adjust to being on the court with fellow elite athletes. With some added lower body strength and a full off season I expect JJJ to show that he can at least be a rotational big in the future.
Moore and JJJ had to adjust the same to increased skill of NBA players - Moore was a bit more ready mentally because he had more experience/a better mentality for playing as a role player. That Purdue team used him as a spot/off-ball shooter, secondary playmaker and defensive player (Jackson was the primary ball handler and Johnson was the featured offensive weapon)
JJJ went from being the Amare of Purdue (look to shoot the ball whenever you get it, chase blocks on defense, don't worry about anything else) to filling a Taj Gibson type role for Boston (focus on defense, be a screener/garbage man/5th option on offense. Huge mental adjustment that he failed to make in time - regardless of physical challenges.
Physically, he's actually got solid tools for his role. His above average length and mobility at the 4 spot give him a good base for making an impact as a pick and roll defender and rotating team defender - he just needs to embrace the role and master it.