In fact, the management should be criticized more than the manager during this period, as the management hasn't made any sound decisions. They couldn't fill the void left by the manager's departure with a good replacement, and they had a very poor start to the season. Naturally, these results aren't surprising. Fortunately, they announced their early departure and that they would continue with another Bundesliga coach. Xabi's performance is extraordinary. The likelihood of them finding another outstanding coach is very low; it would be better for them to sign a manager who can at least remain in the running.
Every decision may have been carefully considered, and Leverkusen's management has made slightly more erroneous decisions as several players and coaches have left for other clubs. But in football, this is normal, as players and coaches come and go to fill the void each season. What's even more problematic is the lack of preparation for the current situation, which has made the team worse. A new coach is expected to bring change, although he won't be able to do as great a job as Xabi Alonso, but at least being able to compete in the top three would be much better.