Chelsea welcome the Premier League leaders to Stamford Bridge on Saturday in a mouth-watering encounter, with City sitting 10 points clear of Sarri's Blues.
After going 12 games unbeaten at the start of the season – Sarri setting a new record for a debutant Premier League manager – Chelsea have stuttered of late, losing ground in what looked set to be a three-way title battle with City and Liverpool.
Victory on Saturday would launch Chelsea back into contention, but Sarri concedes picking apart a Guardiola side is still something he is working out how to do.
When asked at his news conference how his side could beat City, Sarri replied: "I don't know!
"Against Guardiola I have lost every match, so I don't know. You'll have to ask someone else!"
City come into the clash unbeaten in the Premier League this season, winning their last seven top-flight games, and Sarri believes Guardiola has the strongest team in Europe at his disposal.
"The match for us is really very important," he added. "We know it is a very difficult match against, maybe, the best team in Europe. So, we know it is very difficult.
"It has been impossible to beat them for a long period but in a match everything can happen.
"I think that we are improving, but they are improving also, so it is difficult to recover the gap.
"We started the season well now we have to face some difficulties, it's normal to have some difficulties with a new way of playing.
"We have to try and solve problem by problem, day by day, to improve in a definitive way."
The Blues boss believes the team's recent struggles are the result of a lack of motivation, but urged his players to think about their target of becoming one of the continent's top teams.
"A collective target can give us motivation, and the motivation can give us determination," he said. "I saw that in some matches we lack determination. Like in the last match. I think we were not able to have the right determination to win the match.”
"We need a collective target because we need motivation, determination. I can say that, at the moment, they are stronger than us. But our final target is to be the best team in Europe. So we need to work, we need to improve. We are lucky: in 90 minutes, anything can happen. We know very well it's very difficult, but we have to believe in it.