No question he has been an awful rebounder at just 9.9% but for some perspective, Johnson is 10.5% and Jerekbo and Olynyk are 9.7 and 9.5% respectively. Even Horford is only 11.5%. Only Zeller and Johnson have a higher ORB% than Okafor this year. Mind you, Okafor is well below his rookie numbers in those stats, which isn't surprising given he is coming back from a leg injury (you kind of need your legs to rebound).
He was an awful rebounder last year too. So this whole notion of him dealing with a leg injury is just an excuse. This fascination with Okafor is mind-blowing. Looking at the landscape of the league and you'll see similar type players (Monroe and Kanter) are relegated to bench roles. However, those two can actually rebound.
He was an ok rebounder, not an awful rebounder last year (better TRB% than Horford had last year or this year as an example and his 12.8% last year would be 2nd on the team to Zeller this year). No one is claiming he will ever be like Drummond or Jordan defensively or on the glass, but this notion that he is the worst rebounder in the world is just silly nonsense. Okafor is an ok rebounder when healthy. Not elite, but not going to kill you on the glass either (like say Mark Blount).
That's kinda the point though. Because of his relative weaknesses in other areas, he really needs to excel at the things you expect a true center to do. The ability to switch on the perimeter and limited shooting range would be fine if he was a plus rebounder or a rim protector or you could run your offense through the post with him but you can't.
He really needs to be well above average in at least a few other things besides low post scoring to be a positive. Being an okay rebounder or a middling paint presence (which is being nice to his current game) isn't gonna be enough.
his range isn't that limited. He doesn't have 3 point range, but about 20% of his offensive shots are longer than 10' and about 39% of his shots are from 0-3'. Drummond for a comparison has about 6.5% of his shots longer than 10' and about 49% of his shots are from 0-3', and that is this year as a 1st and 2nd year player those numbers are much more dramatic (Drummond was over 81% of his shots from 0-3' in each of his first two seasons). Drummond has his career best 7% AST% thus far this year his previous best was 4.4% last year. Okafor has been 8.2 and 8.5 this year. Drummond's BLK% this year is 2.8 and his TOV% is 12.5. Okafor this year is 4.2 and 10.8. Now obviously Drummond is a far superior rebounder and always has been (his career low was his rookie year at 21.2), and Drummond has always been a far superior overall defender, so I'm not comparing their overall skills, I was just using Drummond as an example to emphasize that a number of Okafor's supposed flaws, aren't really flaws, especially when you consider that he is a 2nd year big man that is still just 20 years old playing on one of the worst teams in the league (and a team that threatened for the worst team ever last year when he was a rookie).
Put Okafor on a team with some real threats to score and his AST% should increase, the number of double teams he faces will decrease, and he will generally be getting the ball in a better position to score and will get better shots which should increase his efficiency. Put him on a team with better defenders in the backcourt and his defensive efficiency should increase as well. Covington is a good defender but he isn't Crowder and he certainly isn't Bradley and it isn't like Okafor is starting, he is coming off the bench this year and playing with a lot of bench players on the 2nd worst team in basketball. Couple that with him barely playing in the preseason and working himself back into game shape, and it is easy to see why some of his numbers are down this year. Of course many of his defensive numbers are actually up, which is a good sign.