I don't think the management expects a spectacular performance from manager Erik Ten Hag. I think it's enough for the management to be in the title race and finish in the top two. It's important to remember that this is his first time managing Leverkusen. Furthermore, since he's coming off the back of a successful manager, expectations are naturally very high. The management will need to temper this and give him a chance. Unless he has a terrible season, I think he'll have achieved sufficient success for his first year if he can lead the team to the Champions League.
The current Leverkusen team is quite strong, so Ten Hag should definitely perform well with this team. If Ten Hag is not able to perform well at the end of the season with this team, then it must be assumed that the team's performance has been so bad only because of the manager's incompetence. Winning the league title by defeating Bayern Munich in the German league was quite challenging for Leverkusen, and Leverkusen showed that. If Leverkusen can win the league title again under Ten Hag, then the management will plan for him in the long term, but if the results are much worse than before, then he may not be seen serving as Leverkusen for long. Anyway, everything depends on the skills and strategies of this manager, if he adopts the right strategy and if the players can play on the field according to that strategy, then good performance is not possible.