Let's make this clear: Al Jefferson is one of the most skilled, low-post big men in the league. He has had six or seven seasons in his career where he could have probably made the All-Star team, if not for the fact that he's played on weak, bottom-dwelling teams for most of his career.
With that being said, Jefferson hasn't shown that he has the ability or will to lead any team further, which is probably why he's never mentioned as one of the NBA's most elite big men.
He's been posting another great season for the Jazz, but his game is very limited and won't get much better from here on out.
Jefferson needs the ball in his hands to be effective on offense, and the offense usually dies out when he does get the ball. While he's added a decent mid-range jumper into his game and improved his spot-up shooting, he's not a very versatile offensive player when not in the post.
Although he's a prominent offensive player in his own right, his defense has never been one of his strong suits. He's never contributed to a defensive rating higher than 103 in a season (per Basketball Reference).
Additionally, opposing power forwards are averaging a PER of 22.6 against Jefferson, and opposing centers are registering a PER of 17.7 this season (per 82games.com). It's time for Jefferson to go, and the Jazz won't lose much of their future with Enes Kanter developing into a more solid player everyday.