If the management of Bayer Leverkusen do not want the players to leave then they know what to do to stop them from leaving, their manager Xabi Alonso has left the team to Real Madrid and most of the players are in the new because they want to join other top European clubs if they can't get a better replacement for the coach and the number of players that will be leaving the team then they should expect nothing but a bad record for next season. Bayern Munich is fully back to their good performance if Levekusen didn't get good players for next season they may end up not finishing among the best 4.
There are times when teams cannot bear to hold certain players so they have to let them go and sometimes big offers from other teams will be difficult for them to refuse. There are many issues that the new coach may have to overcome because the departure of Xabi Alonso will definitely make many adjustments that the new coach will have to make. Likewise, the players recruited for next season and Leverkusen must see the quality as desired by their new coach. Nobody knows for sure how Leverkusen will perform next season, but my prediction is that they will have a little difficulty in keeping up with Bayern Munich's increasingly improving strength.