To be fair, it can be diagnosed well after the occurrence.
That said, even IF this is true (and I don’t wish concussions on anyone, mind), the blame still falls on the Liverpool (and UEFA) medical team for not properly assessing and removing him during the match AND/OR Karius for not recognising the issues he was experiencing and self-reporting. It is an awful position to be in to either have to take a player off against his wishes or take yourself off in that circumstance but not doing it in either case is still on them. There is no one else responsible (except perhaps UEFA/FIFA, as they still refuse to introduce the extra sub for concussion rule, with safeguards to prevent exploitation, of course).