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There have been plenty of surprises for average teams in the Champions League and it could end up being much worse next season. Leverkusen is playing well this season and they are unbeaten in all competitions, but I still doubt that next season they can repeat the same thing. The Champions League is much tougher and even clubs with more experienced players will experience defeat when playing. Leverkusen has only been a proven rock this season playing quite consistently but we never know how they will play next season.
If they play like this season in the Champions League then maybe their chances of coming out as champions are a little bigger. But for next season, no one knows how Leverkusen will play as stable and consistent as they do now. Leverkusen need to test their abilities next season in the Champions League and we hope they can play consistently in the competition there.
It is always difficult to maintain the same performance in different seasons, there are many teams that fail to meet those expectations. Just like you, I also doubt that Leverkusen can maintain their performance next season, especially after they qualify for the Champions League. Several other teams such as Leicester, Napoli and others have failed in different seasons even though they won the domestic title.
There is another team that I doubt too next season, they are Girona from La Liga. Girona and Leverkusen have been two surprising teams this season, but Leverkusen have been much more consistent and they are almost certain to win the Bundesliga title. I find it difficult to expect these two teams to be able to maintain their form next season, unless management succeeds in building a better squad next season.