Chelsea started the Premier League season with five successive victories, but in Sunday's London derby they were frustrated by a dogged Hammers side.
Sarri's men were dominant, though West Ham actually crafted the better chances, with Michail Antonio and Andriy Yarmolenko both wasting glorious opportunities.
Things are not about to get much easier for the Blues, who face Liverpool in an EFL Cup and Premier League double-header in the next week.
And Sarri thinks table-toppers Liverpool – who have won all six of their league matches so far – are a cut above his Chelsea team.
"We have to face a team, a very, very good team that has worked with the same coach four or five years, I don't know exactly," Sarri said in his post-match news conference.
"They are a step forward than us at the moment. We started to work together 40 days ago and [have had] less time with the international [break].
"For us it is a bit early. We have to work and improve and then maybe in a year it will be the same level as Liverpool."
Sarri is not convinced Chelsea's Europa League trip to PAOK had anything to do with Sunday's sluggish performance, though he does not see why the Premier League cannot do them a favour with match scheduling.
"I don't know, I don't think so," he said when asked if Chelsea were suffering from a European hangover.
"In this period, I don't think so. After seven matches it is difficult in March and April, not now. I don't know why in England with the match away you can't ask for a Monday night. In Italy it is normal."
Centre-back Antonio Rudiger was forced off with an injury in the second half, as he started to hobble around the pitch.
Sarri does not expect it to be a serious problem, however: "I want to speak with the doctor," he added. "I think [it's] nothing serious. Maybe three or four days [out]."