Pep Guardiola suggested he had been "too nice" to his fringe players after the decision to make 10 changes to his team contributed to Manchester City's surprise 2-0 defeat to Bayer Leverkusen.
City lost a home game in the Champions League group stage or league phase for the first time since 2018 thanks to goals from Alejandro Grimaldo and Patrik Schick.
Guardiola admitted afterward that the changes he made to his team were "too much" and hinted that he should have been ruthless in keeping his squad players on the bench.
"Too many changes," said Guardiola. "Maybe it was too much, seeing the result. I take responsibility but I saw them and I like everyone to be involved, when you are a football player and don't play for five, six, seven games it's tough but maybe it was too much.
"Always I like to be too nice and involve everyone. It didn't work and we have to accept it. It was the first time in my life I've done it and it was too much."
The result puts added pressure on City's trip to Real Madrid next month.
Guardiola's team have other games against Bodo/Glimt and Galatasaray in January and will be keen to finish in the top eight and avoid a repeat of last season when they finished 22nd and drew Real Madrid in the playoff round.
"We still have three games and we will see what happens," said Guardiola. "I don't anticipate what is going to happen in the future. Now it's don't talk much, clean our heads and go to Leeds."