Muller slams Bayern's lack of 'guts' in damaging loss to Leverkusen
Leverkusen took the lead after 18 minutes thanks to Munich-born Josip Stanisic, who is on a season-long loan from Bayern.
Wing-back Alex Grimaldo scored a second shortly after half-time and Jeremie Frimpong rounded off the win by going the length of the field to score in the fifth minute of injury time.
Muller, a 12-time title winner with Bayern including the last 11 in a row, was visibly angry speaking with reporters after the match.
"What do I think we were missing? In training we show what we can do much better because we are courageous, because we play football freely," Muller said.
"What we're missing is the guts and some freedom to play.
"Leverkusen were taking risks, looking for solutions and playing football.
"We're playing from A to B, B to C, and nobody plays freely or takes risks."
"There are some consequences that you can see on the pitch and that's why I'm pissed off," he added.
Muller laid the blame for the defeat at the feet of the players, deflecting criticism from coach Thomas Tuchel.
"There are not always clear reasons why," Muller said.
"We weren't there today. There were enough international-calibre players - that's why we don't need to discuss the coach."
Leverkusen's defence, the best in the league, kept star Bayern striker Harry Kane quiet, with the England captain unable to add to his tally of 24 league goals.
Tuchel was at a loss to explain why more was not seen from Kane, telling reporters he was "not exactly sure why we weren't able to bring Harry into the game".
"I don't have an explanation, I'll need to review it again."
The coach said his side "would not throw in the towel" with 13 games to play and would "believe until the end" that they can retain the title.
"All we can do is show a reaction and stay in the race."
Leverkusen boss Xabi Alonso said his unbeaten side still had a long way to go in their bid to win a first Bundesliga title.
"We need to keep going - we're only in February," former Bayern midfielder Alonso told reporters.
"We need to keep pushing. We will try to be ready.
"We celebrated a little, maybe we will a bit tomorrow as well, but Monday we'll be preparing for the next game."
