The 75 greatest players in NBA history, ranked by Player Efficiency Rating (PER)

69.    Al Jefferson-20.56

Nicknamed Big Al, former NBA big man Al Jefferson was one of the very best post players in the NBA during his offensive peak. He was a problem on the low block and shot the ball at a high percentage as a result. He was also a skilled rebounder, helping his cause.

With PER adjusting for pace of play and teams that featured Jefferson as their best player often playing slower due to him being a post player, that also helps his cause. During his three seasons with the Utah Jazz, Jefferson averaged 18.5 points and 9.5 rebounds per game, though they finished in the twenties in pace of play twice. 

When taking that into account, PER rates him far more favorably than other advanced stats. As a result, he had a 21.1 PER over that span, which, as John Hollinger points out in his PER guide, is all-star level. In fact, he had a PER over 20 for six of the seasons that he was a full-time starter, though he failed to make an All-Star team each year.

Based on that, it seems clear that Jefferson was underrated, though he is a top 75 player of all time according to this metric. The truth may be somewhere in between; he was definitely underrated over his 15-year career but it's hard to place him as one of the 75 greatest players of all time.