Even in the midst of an 18-game unbeaten run after City eased past the Blues 2-0 in the Community Shield in August, Sarri insisted Chelsea wouldn't be able to last the pace against a team he considers the best in Europe, AFP reported.
However, the Italian admitted his concern at the scale of his side's downturn in recent weeks as defeats at Tottenham and then at Wolves where Chelsea capitulated from 1-0 up to lose 2-1 on Wednesday.
Those losses as well as a goalless home draw with Everton mean Chelsea is hanging onto its top-four place only on goal difference.
"City are the best team in Europe, maybe the best in the world," said Sarri. "Manchester City are in another category. We have to play and to fight to be in the top four."
It is no coincidence that Chelsea's dip has coincided with Hazard's early season hot streak cooling off.
The Belgian captain stood out as an exception to many players suffering a World Cup hangover. After being crowned the second best player on show in Russia, Hazard scored seven goals in his first six league starts of the season.
At the time, Sarri talked of Hazard having the potential to score 40 goals in a campaign although his previous best for Chelsea was 19.
However, with speculation mounting over whether Hazard will tie his future to the club by signing a new contract worth a reported £350,000 ($450,000) a week or hold out for a move to Real Madrid, the Belgian hasn't scored in 11 games for club and country dating back two months.
Sarri has blamed a series of niggling injuries as a demanding summer has finally started to take its toll.
"It's normal at the moment he isn't at the top of his physical condition," added the Italian this week.
Yet, it is clear that without Hazard firing, Chelsea will struggle to even see off Tottenham and Arsenal for a place in the top four, let alone challenge the unbeaten City or Liverpool for the title.
Strikers Alvaro Morata and Olivier Giroud have just six goals between them in the league all season and after promising a turnaround with a double against Crystal Palace last month, Morata's famously fragile confidence looks shot again.
That dependence on Hazard contrasts sharply with City, which has been without another Belgian, Kevin de Bruyne, for practically the whole season and still barely missed a beat.
The outstanding individual in a collective 100-point masterclass as City stormed to the title last season, De Bruyne has played 81 minutes of league football this season due to two prolonged spells on the sidelines with injury.
In his absence, Bernardo Silva has pounced on the chance to shine in a more central midfield role, freeing up the wings for Raheem Sterling, Riyad Mahrez and Leroy Sane to wreak havoc.
Sane and Mahrez were on target on Tuesday as City won a seventh straight league game 2-1 at Watford despite leaving Sterling and many others on the bench as Pep Guardiola made six changes.
By contrast, Sarri's decision to make five changes was questioned after defeat at Wolves.
"I don't think it depends on the changes; it depends on the mentality of the team," said Sarri. "We can lose a match, but not without a reaction."
Now he needs a reaction against the toughest team of all.