Not a lot in my opinion. He had a fantastic start to his Leverkusen career but since hasn't really looked amazing.
That was my impression of him too, albeit mostly just based on stats. In 20/21 he looked statistically amazing in possession (pretty much in the 99th percentile across the board in both passing and carries) while also very solid defensively. I ended up watching a few Leverkusen matches just to see him and he was very impressive, so he became my ideal defensive signing at the time and looked the perfect signing for a manager who wanted to really dominate the ball and play out from the back. His defensive partner Tah also looked very good in the games I caught.
However in the last two seasons his stats have come back to the pack a bit (still very good but no longer insanely good), and in the one or two games I did watch last season I didn't think he was quite as good as he was a few years ago. Obviously that is based on a very small sample size though, and the change in stats could easily just be a change in tactics from the team.
I'd still be very happy to sign him, but I'm not sure he's my clear-cut first choice anymore.
