My hunch is that he's the worst-rebounding PF in the league
His Total Rebounding Percentage is a little under 10%. Last year, in fewer games (before the knee injury), it was a little over 10%.
That puts him in the territory of guys like Patrick Patterson, Boris Diaw, Jerami Grant, Mirza Teletovic, Mike Scott, Josh McRoberts, and Anthony Tolliver.
That's better, by the way, than guys like Jae Crowder, Danilo Gallinari, Demarre Carroll, and Harrison Barnes.
So for a small / quick PF, it's not great, but it's not nearly as bad as you make it out to be. I'd hope his rebounding would improve somewhat as he continues to come back from the knee injury so that it's more in line with guys like Luol Deng, Matt Barnes, and Rudy Gay.