And Gundogan's shot from the edge of the area then had too much power for the world's most expensive goalkeeper Kepa Arrizabalaga to compound Chelsea's woes. Missing out on the continent's top club competition would leave the Londoners needing to offload stars to avoid Financial Fair Play breaches.
As City rubbed it in by blasting out Madness anthem "One Step Beyond" over the Etihad sound system - which the Blues themselves usually play at home victories - speculation was well underway that Sarri had one step out of the Chelsea door. "The only thing I can say to the fans is sorry, because it's not acceptable, and I feel sorry for all of them".
It was Chelsea's third consecutive away loss in the league, following a 2-0 collapse at Arsenal and 4-0 humiliation against Bournemouth.
Asked if this defeat made him fear that his style of football can not work at Chelsea, Sarri replied: "No, because today I didn't see my football".
Lampard, Chelsea's all-time leading scorer with 211 goals, said he was focused on doing his best for Derby and backed Chelsea to bounce back quickly from the defeat, which pushed them down to sixth in the table.
Kante said on chelseafc.com: "It's hard to explain".
Sarri was asked if he was expecting a call from owner Roman Abramovich, but the answer pointed towards the power structure at Chelsea.
Strangely, Aguero's first major involvement in the game was arguably one of the worst misses of his career when he inexplicably stroked the ball wide of an open goal from all of two yards with the score at 1-0. "I don't know why".
"I think this is a crucial moment in the history of this club", Mourinho stated.
"But I am not able at the moment to understand, because during the week my feeling was really very good".
City eased off for the remainder of the first half, and Chelsea striker Gonzalo Higuain even managed to test goalkeeper Ederson with a sharp volley.
"He's [Maurizio Sarri] turned Chelsea into the Arsenal team that used to get battered for the last seven, eight years away from home in big games because they're so weak".
"We made a lot of mistakes against the wrong opponents".