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I must say Bayern Munich defense far from what it was before to be honest, this is an obvious fact.
Anyway I think the beat way to defeat bigger teams is with a good defence and a good tactics whereby if the underdog can maximise their chances can get a win or at least a draw against any top team. Feyenoord pulled this off all eyes will be on them in their next match though which is against Lille, the two teams are a dangerous one as an underdog in this tournament.
Bayern Munich's defense has been bad for a while and it was their weak point although I have to say that both their offense and their defense is their weak point for now. Now you will come up with the number of goals they scored in the Bundesliga and how they beat Zagreb 9-2, but all of this is irrelevant if against the best of the best teams they can't do the same or keep a clean sheet.
The game against Feyenoord was about the best example you can get to see how bad their defense is. The same happened when they played against Barcelona. It shouldn't be possible for the opponent to use one long pass deep towards the box of Bayern and immediately score with one or two additional touches by the striker. That is not acceptable because Real Madrid or Barcelona will destroy them that way. Barcelona has done it already.
I don't know whether it is the speed of the defenders, I think it is more how they are positioned and that the defenders don't have the understanding how to position themselves defensively within the game that Bayern is playing.
Their attacks are so ill-considered sometimes. You can literally observe how they can't find the best idea and how to execute the attack. it was the same last season.