Pep Guardiola admitted Manchester City were "really, really poor" as they slumped to their first home defeat in 15 months at the hands of Basel.
The runaway Premier League leaders were beaten 2-1 by the Swiss side after making six changes for the second leg of their Champions League last-16 tie on Wednesday.
The loss was not enough to prevent City progressing to the quarter-finals 5-2 on aggregate but Guardiola was nevertheless frustrated by the drop in intensity.
"The first half was quite good but in the second half we forgot to attack, we forgot to play," said Guardiola, whose side last lost at the Etihad Stadium against Chelsea in December 2016. "We just passed the ball.
"Passing the ball means passing to move the opponents and attack - to pass for itself is nothing. The second half was really, really poor. We just passed for ourselves and that is not football."
The result meant City have still not won both legs of a knockout tie in the Champions League, but Guardiola took comfort from the obvious positive.
"From tomorrow we will be happy to be in the quarter-finals for the second time at this club," he said in his post-match press conference. "We are new in that position, so of course we are so happy for that.
"We will see the draw. When you are in the quarter-finals, at that level, all the teams will be so tough and demanding. We will try to be ready."
City started brightly and took the lead through Gabriel Jesus after eight minutes following good work by Leroy Sane and Bernardo Silva.
Jesus was starting for the first time since suffering a knee injury on New Year's Eve and the goal was his first since November.
"It was important he played 90 minutes," said Guardiola. "He fought but he needs rhythm. When you are out for a long time you need the rhythm to come back. Of course the goal he scored will be good for his confidence."
Basel hit back to equalise nine minutes later through the impressive Mohamed Elyounoussi, who later set up Michael Lang's 71st-minute winner.