The "back to the basket/immobile" big is now a dinosaur in the NBA. Look at Dwight Howard, in a 5 year period, he went from being a MVP level player, to being salary dumped by the Hornets lol.
Jah honestly just came into the league during a transition period. If he's playing in the 90's, he's a starting center, getting buckets and clogging up the lane.
I believe he will still find his niche' as a coming off the bench guy who can carry a 2nd-unit offense.
As for the rebounding...he SHOULD be better than what he has shown in the NBA. However, he gobbled up most of his college rebounds by standing under the basket and being bigger than everyone. In the NBA, center's have to guard all the way out to the 3 point line, and he's not bigger than everyone. He has really struggled to adjust.
Foot-speed and attitude. He fixed those 2 things, and he will make a lot more money in the NBA.