Defense is indeed something that Dortmund needs to fix before they start the new season, I see Dortmund's pre-season matches are not too good they always concede goals from weaker clubs, it's useless if they have a good attack line but their defense is weak then their opponents will know their weaknesses and create strategies to defeat Dortmund.
However, I realize that if the coach does not bring out the best players in this pre-season match I think Dortmund still has a good defense let these matches be evaluation material for the coach to solve the problems that occur in his club.
Despite pre-season matches not fully determining the form of a team, the truth is that many teams that perform well in pre-season often do well in the main season. If a team uses its full squad during pre-season and their performance is poor, it's unlikely that they will perform better when the season starts. I haven't been watching Dortmund's pre-season matches, but based on some comments I've seen, it seems they have issues with their defense. If that's the case, the head coach needs to take quick action and find a solution, perhaps by signing great defenders. Sometimes, it's through pre-season that one can start predicting some teams performance.
I disagree about that all together. I think friendlies means nothing and teams do not deal with anything serious and do not play that well at all. Players don't care, they are just playing like it's a practice game and they are just jogging, the mindset of "I must win this game" is not there at all, and this is why we do not see good teams end up getting great results against lower teams at friendlies all the time. What teams are trying to do in these games is to just test players and their fitness, that's it, if a player is fit that's good enough, and they try to see any new transfers and how they are doing too.
So, if you do not play any friendly at all, and suddenly put the players in a real match, they are going to be out of breath within 15 minutes, no matter how professional these players are, and no matter how great their body might be, if you do not give them some friendlies before the season starts, they will not be in shape, even an intra-squad friendly won't help you, you need to play against some other team. We will see teams turn it around when games start to mean something, I can guarantee you that. We are going to end up with Bayern, Dortmund and many others, end up getting their wins as usual.