He hasn't proven but he has shown great signs for it. Enes can always be in top 20 at rebounds in the NBA and his ceiling is up to the top 5.
He was in top ten in his rookie year and top 25 in his second year. The most plausible reason for his drop in his third year is obviously his injury. He was always an elite rebounder all his career long including his rookie year in the NBA. So I believe that there is no reason for him to get it back again.
I think using his injury as an excuse for his rebounding is wishful thinking.
Kanter has NEVER been in the top ten in the NBA in defensive or total rebound rate category.
OK, my rankings weren't for Reb Rate category, they were per 48, I read your post and wrote that in a hurry. But they are not totally worthless rankings either.
Still, with very slight adjustments, he was in top ten in the REBR category too in his rookie year. Actually it is, if you only use a better pt variable for players qualification. ESPN requires only 6 mins pt over 82 games, make it as 10 mins and 30 or 40 games minumum and he is 10th or 12th in his rookie year. Furthermore, being 17th in ESPN's and Hollinger's list isn't bad either for a rookie.
He's been in top 10 in offensive rebound rate, which isn't nearly as important as defensive rebounding. Kanter was 59th this year in defensive rebounding rate.
Do you know where he ranked the previous year? You know, before the injury? 89th. His first year was his best year both in raw number and ranking, and he was 37th in the league.
He's NEVER been a great defensive rebounder, pre nor post injury.
Wow, offensive rebounding is NOT NEARLY as important as defensive rebounding? It is definitely debatable for certain aspects of the game, that which one is more important, the defensive rebounds or the offensive rebounds. But the result of that debate can not be "not nearly important" for either one. To me the offensive rebounding is at least equally important and valuable as defensive rebounding, if not even more since the offensive rebounds and having that ability is rarer. And for the sake of the argument, pulling offensive rebounds is the harder one out of the two. Because the defensive rebounding is much more of a team work whereas you're mostly on your own in offensive rebounding.
So, I don't find it much genuine to focus solely on the defensive rebounding all of a sudden and dissing the offensive rebounding for the sake of your argument and to make a basis for it.
Anyway, for the Pre-NBA of Kanter, no one has any doubt whatsoever about his more than great(elite) rebounding. For the NBA, I believe everyone would agree that he's a solid rebounder all around and a tremendous rebounder on the offensive side. As for the defensive side, like I said, it's much more of a team issue. The Jazz were always decent when it comes to rebounding. The rankings you gave(14 and 16) for the last year were actually the worst of the team for the last recent years. Before that the Jazz were 12th at REBR and were 3rd in the rookie year of Enes. That fact can also be interpreted as the team has decent rebounders and players share the rebounding. Plus, Enes was never the primary rebounder on the team, not yet at least. I can't say anything for the last season, because I haven't watched it, but the previous seasons I could swear that Enes was boxing out much more for his teammates than he was doing for himself.
Nevertheless, I never said he's proven as a great rebounder in the NBA yet and it's an undeniable fact that he regressed rather than progressing at rebounding. I know his boxing out got broken in a weird way(we joked a lot about it as, he found religion so he doesn't recline his *** to other men anymore) and he lacked the effort on the glass. But I'm standing behind my opinion that his upside is the elite level at rebounding even in the NBA. He has it at least as his potential and has shown enough signs for that he can realize that potential. Elite won't happen for sure if he focuses on the offensive side of his game more in the future but I've no doubt that he'll be always solid and can really be great if he wants it.
Edit: Maybe we should ask, why? A player that dominated everyone with his rebounding until he came to the NBA, and showed in his rookie year that he can do a great job at that in the NBA too, did not progress and contrarily regressed. A player who is a force at offensive rebounds in the top basketball league in the world just can not pull defensive rebounds as much as successfully. I'm sensing there is more things to it and it's not only and entirely on Kanter's shoulders.