Unless he can increase his rebounding by at least 60% I don't see the potential to be a better player than Gortat.
Is Erden a bad rebounder? Statistics may say yes right now, but I'm not so sure. He has an injured shoulder and an injured thumb. That is going to limit his ability to rebound with two hands, severely. I think this is why you see him tapping rebounds...one handed...toward team mates a lot more than grabbing and pulling them down. If you could count the taps to his team mates as rebounds you'd increase his average by two or three a game.
I've been watching him during games and it looks like he has the tools to be a good rebounder. He knows how to block out, gets overpowered sometimes, but he clearly knows how and tries. He is better about not getting stuck to his man and going after balls outside his "zone" than quite a few centers in the league. He is still tentative, at times, and a bit slow on rotations and positioning, but I write that off to being a Rookie and someone who doesn't speak the language very well yet. He needs to improve his strength as he is too easily shoved off the blocks and backed down under the basket. He hasn't shown great leaping ability, but that's no surprise, but he has shown good mobility, willingness to run the floor, good hands under the basket, and the ability to take instruction and learn.
Erden is making a lot of Rookie mistakes, he's playing hurt, he's playing without a complete understanding of the offense and defense, without enough experience at playing with these guys to always anticipate what they are going to do, and without a good grasp of the language everyone else is speaking
on the court. Personally, I think he's doing pretty [dang] well!
Oh, and he's 24...like that's old? Come on! A four year college player enters the league at 22 or 23 and keep developing for another few years before hitting their peak. So, Semih is a year older? There is nothing to stop him from continuing to get better each year for several more years.