As a Turkish person I might be a little biased towards the whole Arda Guler situation, but I have to say the kid is only 18 years old and from the images that we have seen so far, from the day he got to the team versus the latest pictures that we got of him, we can clearly see that he has put on some serious amount of muscle on his legs, not that huge of a difference on his body but his legs definitely got huge compared to how it was just six months ago. That could be one of the reasons why he got injured so frequently, they know that he is talented, but they are overloading him with extensive training that pushes his body to limits which injures him a lot. The moment they let go of his training situation, he will probably get better and won't get injured as much.
He is only 18 years old, at his hormonal peak, he won't have as much testosterone as he has right now. I think that if they are pushing him and training him harder, that's good right now because when you build some muscle in teen ages, it's easier to keep that. I don't know how if it's a broscience or not but that's what I know. I remember when I was a kid and watching Lionel Messi's performance in matches, I noticed that he gained huge quad muscles and I was always thinking that it was making him slow and was requiring more energy.
Btw I don't think big quads are the reason of his injuries because of his current age.