Goals from Pedro, Morata and Alonso gave the Blues victory against the North London club at Stamford Bridge.
Mkhitaryan and Iwobi scored for the visitors.
But Sarri believes the Blues won’t be title challengers this term unless they improve defensively.
Sarri’s men have now won their last two EPL games to top the league table with six points.
“We are not title challengers, not in this moment,” Mirror quoted Sarri as saying at the post-match conference.
“We have to improve in the defensive phase and I’m not talking about only the defenders,” Sarri added.
“We did very well for 75 minutes — 15 minutes at the end of the first half was horrible.”