Most of it was not mental.
He has no go to offensive moves (no step back or pull up or ability to get to the rim against smaller defenders nor ability to finish at the rim) he’s got an average handle, he’s slow moving off the ball, and is an average athlete which puts him at risk on defense.
In fact, he had about the worst offensive game of any P5 and was still given a starting job.
http://barttorvik.com/playerstat.php?link=y&sIndex=5&sortToggle=1&cvalue=High%20Major&year=2018&top=351&start=20171101&end=20180501
Over time he could develop a go to move and improve handles, but that is why he needed the year off.
But, this wasn’t mostly mental.