Someone here made me to realise that he failed his medicals because of the injury he has made to be frank that's a good reason for a club to reject signing such a player even if it's a loan deal because the team will be the one to split the players wage with the parent club. Now I understand better and even if I'm the owner of a club, I won't buy an already injured player only for the club to start taking responsibilities of an injury he got from another club.
Let's not forget that the knee injury didn't happen this season the injury has happened in the past and he has been using it to play for his club Bayer Leverkusen and he has not been sidelined for a long term injury, AC Milan rejected him because they are scared that the player may be injured again and it may not be of good benefits to them but that he is currently facing injury issues just a clearer understanding.