Main problem with Hez is that he jumped several steps over in his career so far. He was launched in the top basketball (Barcelona) way too early. He was only 17 at that moment, playing for the B-team and never actually got real role in the first team. He showed his brilliant skills regularly, but lacked from consistent minutes and serious role in team. Suddenly he found himself at the 5th overall pick in NBA. This young guy will just have to learn soooo much that he hasn't, couldn't or didn't want to so far.
My opinion is that he's just not ready for NBA at the moment. Back to Barca or any other Euroleague club for 1-2 years would help him develope with some real minutes. Otherwise, his talent will be wasted in NBA.
His teammate Šarić from Sixers is a very good example. He was also pushed for years to go to Draft, positioned himself first as a very good European player in Cibonam, especially in Anadolu Efes where he played some huge minutes and had a very important role. Result: We are watching very solid NBA player, important rotation part, for now. Hi is tacticly, mentaly and phisicaly much more ready for NBA than Hez.