Leverkusen has some big problems to solve. Tah is gone, Wirtz, Frimpong, the coach Alonso of course, I don't know who else will leave, but this is serious change going on.
In addition to the players and coach Xabi Alonso, now their mainstay striker Patrik Schick is also rumored to be moving to an English club, because currently Arsenal, Chelsea and several other English clubs are interested in bringing him in. He is a young player who is hungry for goals, he has performed very consistently, so the hope will be better if Leverkusen chooses to keep him.
You name it and this is not the end of the story yet. We are still in May. This is going to be a monster task to put together a team that is nearly as strong as what Xabi Alonso has put together. But I think this is the biggest fear that Leverkusen now has for a good reason. One player after another leaving the club and they have to find these super talents again. Not one of them, but probably five of them.