I think his flaws were pretty apparent right from the start. I think one big problem is that this organization and JBB just didn't seem worried about correcting his flaw or any players flaws really. They just leaned in heavily on the players strengths to the point that the player didn't think they needed to work on their flaws at all.
We now have 4 examples of highly drafted players who just didn't develop much under JBB after their initial jump from the NBA game slowing down for them. Sexton, Garland, Okoro, and Mobley. I think everyone thought each of them should have had higher ceilings and better progress than they have shown so far. Garland seems to regress from his all star year. Okoro and Mobley have added 3 points shots but are basically the same players they were their rookie year.
I don't expect Garland, Okoro, or Mobley to immediately make a huge jump under a new HC. I think their development have been stunted and all I'm hoping is their development can be restarted under a new coaching staff. It might take multiple season for any one of them to make another jump if there is any correctable development left in any of them.