The Champions League holders confirmed top spot in Group G with a dramatic victory, having trailed 2-0 at half-time.
Louis Openda scored twice for the Bundesliga outfit, leaving the Etihad stunned to silence and Guardiola with plenty of work to do.
He changed Ruben Dias for Nathan Ake at half-time before withdrawing Grealish and Kyle Walker for Jeremy Doku and Julian Alvarez before the hour mark.
Moments after the second wave of substitutions, Erling Haaland became the quickest player in Champions League history to reach 40 goals to spark life into the hosts.
Phil Foden assisted Haaland’s strike and scored a goal of his own in the 70th minute to draw City level before Julian Alvarez netted the winner with three minutes of normal time left.
In his assessment, Guardiola highlighted Rico Lewis and Foden for praise. And the Catalan coach gave Doku a special mention for influencing City’s game-winning reaction.
“That’s the Champions League [City’s first-half performance],” Guardiola told TNT Sports after full-time.
“Leipzig are always a team that qualifies for the last 16 the last five or six years. We conceded too much defensively. You have to win the actions, and we didn’t do it.
“We had to be as relaxed as possible, and we reacted really well. We are first in the group, and I am very satisfied with the reaction at the end. The team runs and fights and has spirit.
“We changed with our press. Rico [Lewis] was again exceptional, and Phil [Foden], in the final third, had decisive movements. I think Jeremy [Doku] changed our rhythm.”