The Bavarians were on a winning run of 10 games in Europe's top competition before being beaten by City after leading 2-0, with Muller claiming that he and his teammates got "complacent" with their comfortable lead.
"I think we showed our human side today," he told UEFA.com. "We were too careless at times. It was not as if we gave away chance after chance, but we did not look good on any of the goals we conceded.
"It is no secret that City are always good for a goal, but we got off to a great start and put them under a lot of pressure. After that we took our foot off the pedal.
"We need to get that winning mentality into our heads. Maybe it is understandable that we got a bit complacent after so many wins but we are professionals, so that kind of thing is not acceptable."
Despite the defeat, Bayern qualified from Group D in first place.