Somebody's really gotta teach him the defensive stance, positioning, his weaknesses and what he's gotta do.
1-His lateral quickness especially on his left side is one of the worst so he's clever enough to get he's slow, he's trying to give lots of spacing to the opponents to not getting abused and generally it does not end up good result.
2-At the low block, still he makes same mistake by not trying to using his body to prevent opponent's positioning so it 's generally so late for him because of his wingspan etc.
3- On PnR defense i really do not understand why he shows up so high with his slow foot and then try to cover up but never achieves (Corbin's bad defense strategy, look at the Portland defense for PnR not showing up high and San Antonio's and Memphis's press to drive opponent to the baseline instead of comfortable angle)
4- Back-court defense problems: It's so diffucult in the NBA to defend quick, athletic guys coming over you by big fellas without fouling ( Jefferson and Burke is so weak to stop or make opponent slowier)
His offense is get better and better match by match. Better midrange shooting, sharper post up moves, good offensive rebounding numbers and a lil bit dribbling ability