I don't know what problems Mainz is having. But I see that this is Union Berlin's worst season since they were called a dark horse with increasing achievements in every season. Many say this is because they recruited Bonucci who was considered old even though he had a lot of top level experience. If you look at Union data, Berlin does have problems in their defense this season, but that is not entirely the fault of one player. Currently, Union Berlin as a whole must improve their defense because several times I saw them concede after taking the lead first.
In seeing a team that most disagree if it is associated with just one player because on the field we see 11 players who appear to work together, yes despite the shortcomings I think Bonucci is a pretty good player and has experience, but is currently no longer in Union Berlin if I'm not mistaken.
Why Leonardo Bonucci is still linked with Union Berlin, Leonardo Bonucci is not playing for Union Berlin at the moment and so far Leonardo Bonucci has played for Fenerbache. Previously, Leonardo Bonucci only had 10 games for Union Berlin before moving to Fenerbache, but his performance was not too bad as an experienced defender, he also scored 1 goal in 7 appearances in the Bundesliga.
So far, Union Berlin's performance has been very poor. Apart from the weak defense, goal productivity has also fallen. Union Berlin have conceded 35 goals in 20 matches or an average of 1.75 goals conceded per match, while they have only scored 19 goals in 20 matches or an average of 0.95 goals per match. Of course those are the statistics and form of a poor team performance for them after qualifying for the Champions League last season.
Yes we can see in their defense and midfield players are not so solid this season which makes them always careless and my analysis is that players often rely on other players for certain tasks and often miss communication, I am a little suspicious that maybe some players have problems with other players because the decline in performance experienced by Union Berlin is very drastic this season, what can trigger this is a misunderstanding that can destroy the chemistry in the match.
What Union Berlin needs now is a better and more qualified squad. They can never keep the team's performance the same every season without major investment in building squad depth instead of bringing in more players in the same season. There are 13 new names in the Union Berlin squad this season, of course this could make them fail to get a good performance considering that all these players need time to adapt.