1 - In all the friendly matches they have conceded a goal
2 - They have played 2 games in the Bundesliga, conceding at least 2 goals in each game
3 - They have only signed 1 defender this season: Jeanuël Belocian
https://www.transfermarkt.pt/bayer-04-leverkusen/startseite/verein/15
In just 2 games they conceded 5 goals. This number of goals conceded should be at least 10 or 15 games, normally for a team that is a candidate for the title. By this I mean that the Leverkusen coach has reason to be worried, because it is possible that they will not even finish in the 3rd position. This weak defense will also be a problem in European competitions.
Not expecting much from Leverkusen this season honestly. After a belter of a season, a quite unbelievable one, you start over-complicating things. That's what's going on over there right now, and it's not their fault. I understand that they have built a certain expectation around them, but always living up to the expectations is difficult, for everyone.
Leverkusen already had tough games in front of them, and are having a hard time keeping up. They still would be a top team and would stay top 4 in the league till the season ends, but they don't have all that it takes to go for the title again. A few offloads and incomings probably in the winter, but that does not really change much, does it? They needed to strengthen a lot more in areas they need strengthening because even after an unworldly season of football, there would be areas you need upgrades in, but I don't think Leverkusen were able to get that upgrade.
Now they are also going to be playing UCL in a few weeks, and there is always a flow of league matches around them. So it will be really interesting to see how they cope up with that, or were they only a one season wonder? because more often than not, teams who get crazy success in a season, struggle really hard to keep up with that level of consistency. So this will be the real challenge for Leverkusen, which I am really excited to find out.