Leverkusen would not have regretted losing any other player as much as they regretted losing their manager. They will not get such a manager for money. The current manager has a poor background, so even after giving him the job, the management may be skeptical about how the team will actually perform under this manager. Leverkusen performed well in the previous season and they did something like defeating Bayern Munich. Now if this manager cannot maintain the same level of performance of the team, then it will definitely not be accepted well by the management. Let's see if this failed manager can do anything good for the new team.
Seriously, it will really be very hard for Bayer Leverkusen to get a coach like Xabi Alonso. I don't think it will be long for them to start regretting the absence of Xabi Alonso. Long players can never be like a great coach. What Xabi Alonso has done for Leverkusen, I don't think they will have a coach who can do that for them in the long term. A coach who carried Leverkusen from low performances and made them win the league unbeaten.
I am not underrating Ten Hag like that, but I don’t think he can maintain Leverkusen's performance. He is not that bad a coach either, but which big player may wish to play under him? Look at what happened to players who played under him at Manchester United most of these big players lost their form. So, even if Leverkusen gives him an opportunity to sign his players of choice, it will be really hard for him to sign big players unless he builds players himself. I don't think Ten Hag believes in building players based on the signings he made in Manchester United. I just hope Leverkusen doesn't later regret Ten Hag as coach.