You could go anywhere on the Internet and see how difficult it must be to guard Dirk Nowitzki. Basketball-Reference considers him a legend. ESPN always ranks him among the best at his position. NBA.com has stats that will melt your mind. Heck, there are even plenty of articles on this site that make the same case.
It’s one thing to hear it from analysts, though, and another to hear it from former players. Big man Nazr Mohammed, a veteran of 17 NBA seasons and eight teams — he most recently played with the Chicago Bulls last season — wrote a column for The Players’ Tribune where he ranked the six toughest players he ever had to guard.