It's a big mistake from the management of Bayer Leverkusen to let Xabi Alonso leave the team after losing their key players that helped them to win the league, they may not have had what it takes to keep Xabi Alonso for many more seasons but allowing him to leave when they have lost so many players will definitely affect the team because the new manager will start a fresh to build the team and they may not be able to contest for the league title next season.
What is a good manager without good players. It is also a good way of allowing the new manager Erik ten Hag choose his squad according to his formations. Xabi Alonso wasn't demanding anything the club needed for him to stay, it was a fair departure from the club and i don't see it different from this, now it is left for Erik ten Hag to retain their position as competitors for the Bundesliga title. My problem is that people will judge Erik ten Hag too early because of how bad his season went at Manchester United, quality time should be given to him since it is about rebuilding the team.
I will stand with GiftedMAN they shouldn't have allowed Xabi Alonso to go since the team has Lost there good players, this action they took will seriously affect them in there performance, yes it is true that it takes good players for good coach to succeed but some coach can't even use good players talk more of bad players, if Xabi Alonso was still in Bayer Leverkusen I'm very sure he will know how to manage the players he have and they will succeed.
For me I don't see Erik ten Hag as a coach that can develop or lead Bayer Leverkusen to a good place talk more of lifting a trophy, except he has improved, looking with how he coach Manchester United I don't think he can coach Bayer Leverkusen well however maybe he has really improved and can develop them well so let's just sit and watch how everything will go.
Good luck to Erik ten Hag.