I don't know what you think, but I think Leverkusen will end the season with a very bad performance under this manager. The players in the team have not changed, only the manager has changed, and I don't know why the team's performance has deteriorated a lot because of this manager change. The management and the fans are definitely not satisfied with the manager. Now if Leverkusen consistently performs poorly in the first few matches, what action can the club take against the manager?
Maybe the club will try to find a good manager, but a manager doesn't really want to take charge of a team in the middle of the season, so if Leverkusen can't find a good manager, maybe Ten Hag will take charge of Leverkusen this season, but Leverkusen will definitely change their manager next season.
It is clear you are unhappy with how Leverkusen is playing since new manager took over. It is common problem in football new coach can mess up team flow even with same players. It is frustrating to watch your favorite team struggle and it makes sense that you are worried about manager effect. You made good point about club being under pressure to make change if team keeps playing badly. As you said finding good manager in middle of season is very difficult and your idea about Ten Hag is interesting. It is tough situation and you are probably right that permanent change might not happen until end of season.
Ten Hag's background is bad, that's why we can't take Leverkusen's defeats under him as normal, but if any other manager had taken charge of Leverkusen and Leverkusen had performed relatively poorly, we would have thought that the manager was new to the club and that's why it takes some time for his team to perform normally. When Xabi Alonso took charge of Leverkusen, the Leverkusen team seemed confident and they played their best on the field, so why Leverkusen's players don't seem confident since Ten Hag took charge, they can't perform like before, is always a controversial issue.
Although they criticize the manager, those who are Leverkusen supporters will not want their team to perform badly, so Leverkusen supporters certainly don't think of any alternative to Leverkusen winning. Now, if the team performs poorly under the manager, then the management will be forced to fire this manager under pressure from the fans.