While City still cruised into the quarter-finals on the strength of a 4-0 win in the first leg, the Spanish boss was not pleased with how his side performed in a second half that saw them surrender a 36-game unbeaten run at Etihad Stadium .
The Premier League leaders were largely dominant in terms of possesion on the night, completing 978 passes - the most recorded in a Champions League match since Opta began keeping the stats during the 2003-2004 season - but even that wasn't good enough for the former Barcelona boss.
"We are happy to be in the quarter finals, we are so happy for that," he said.
"Second half we didn't attack and forgot to even pass the ball... the second half was really really poor."
Guardiola singled out just one player, 17-year-old Phil Foden, who broke a Champions League age record by starting the match , for praise during the second period, which the manager believes lacked any form of aggression from his team.
"Phil was good and in the second he was perhaps the only one who tried to be aggressive," he said. "It’s good for the rhythm of the future."
With the job done in the last 16, City will now turn their attentions to the Champions League quarter-finals, and Guardiola knows the next round will be much tougher on his team.
"The difference in the quarter-finals, in that level all the teams will be so, so tough and demanding," he continued.
"We will try to be ready and to come back because we did it in the recent games."
City hold a commanding lead in the Premier League and will find out who they will face in the Champions League quarter-finals on March 16, with Juventus, Real Madrid and Liverpool the three other teams to have already confirmed their place in the last eight.