The Blues handed Pep Guardiola's side their first loss in the division this season, with N'Golo Kante and David Luiz scoring either side of half-time at Stamford Bridge.
Sarri's side are still seven points behind City after 16 games and eight behind new Premier League leaders Liverpool.
And the Chelsea coach remains insistent that his team cannot maintain a sustained title challenge this season due to his view that City are the continent's finest side.
"As I said, always as you know, I think the gap is impossible to cover this kind of gap only in one season," Sarri told a news conference.
"One match - you can play one match, of course. Then in the competition of 10 months it's very difficult to cover the gap. I think they are, at the moment, the best team in Europe.
"We won today. But we have to say we were a bit lucky in the first 25 minutes, because they could score. A little bit lucky in the first half and very good second half."
In his previous three meetings against Guardiola teams, Sarri had never been on the winning side.
As pleased as he was with the victory, Sarri wants his Chelsea side, who lost at Wolves in midweek, to carry their form into the rest of December, when they will face Brighton and Hove Albion, Leicester City, Watford and Crystal Palace in the league.
"I'm happy for the win because it's difficult to win against Guardiola's team," he added.
"I know very well they are a step ahead now. We have to work to cover the gap.
"We are not sure to be able to cover the gap. We have to try, of course."