Martin Hinteregger scored off two set-pieces in three minutes, meaning questions will be asked of the Bundesliga leaders
Thanks to goals from Leon Goretzka, Thomas Muller and Robert Lewandowski, Bundesliga leaders Bayern looked comfortable with a 3-0 lead after 50 minutes at the Allianz Arena on Saturday.
However, they were caught napping at two set-pieces in the space of three minutes, with Hinteregger allowed space to bundle a volley past Manuel Neuer before powering a header into the back of the net.
Alphonso Davies quickly put Bayern back in control before a comical own goal from Hinteregger secured a 5-2 triumph for Flick’s men.
The Bayern coach hopes to see better defending when his team take on Dortmund, who sit four points back in second, at Signal Iduna Park on Tuesday.
“After 3-0 we switched off. Frankfurt set very good standards, we knew that. We didn’t look good there. We have to talk about it,” Flick told Sky.
“We want to keep a clean sheet against Dortmund. It will be difficult because Dortmund have a very good team. We all hope for a good game of football.”
Muller, who registered his 17th assist and seventh goal of the Bundesliga season, acknowledged Hinteregger is a significant threat from dead balls but felt Bayern should have been more alert.
“You never know how to get into games without fans, but I still expect us to go to the maximum,” said Muller.
“Hinteregger is a very good player in set-piece situations, but if the ball comes in and no one feels responsible for it, of course we have to talk about it.
“We have made it our goal to have three victories this week. This is a very crucial week for us. We are marching, Dortmund are marching, we can look forward to Tuesday.
“We want to bring the championship to Munich, where it belongs.”
Goretzka added: “I think Hinteregger is one of the strongest players when it comes to set-pieces, but, of course, we have to talk about it. That is not possible. It does not work that way.”