The Germany international was not amused with the Bavarians' performance, but stressed that they did not underestimate their opponents
The Bundesliga giants were beaten 3-1 in their second Group F match, but Badstuber has stressed that die Roten did not underestimate their opponents.
"We don't know how we conceded against that team, but we just weren't aggressive enough in one-on-ones to win balls, and they were just clinical with their counterattacks, and that can be enough," Badstuber told Uefa.com.
"They scored the opener, but if we had then taken the chance we had to score, it would have ended differently. After that, we were trying to equalise, and their second goal was decisive.
"We take every opponent seriously. We knew what to expect here, especially after they beat Lille 3-1 away.
"Now we just have to stay calm. We shouldn't lose our heads – we know what we are capable of. We have to win our next home game [against Lille] and then it looks different again."
Bayern resume Bundesliga action on Saturday when they host Hoffenheim.