Let me put the Lopez thing in perspective, you want your Bigs to be averaging around 6 - 8 defensive rebounds, minimum, a game. And the rest comes from Offensive Rebounds.
Look at Marc Gasol and Zach Randolph for e.g.
Marc has never had massively high rebounding rates. Why? Because he plays next to Zach. Which means he plays a lot in the high post, while zach plays a lot in the low post. Zach is an offensive rebounding monster, and thats what puts him well over averaging double digit rebounds.
But they both average around that 6-8 DREB a game mark. Which is what makes them a good rebounding front court. Not great, but good, because Marc's averages are down a bit.
Marc's numbers are down this year, but he's still boxing out, and his number will get back up there.
Lopez is trending the complete opposite direction. And he has never averaged over 6.
Why is it something that people berate him for? It's very hard to be a winning team when you get out rebounded all the time and give up a ton of second chance opportunities.
I'd like to check it against 2nd chance points / opportunities, but i can't find that anywhere at the moment.