Sarri was particularly concerned about his team's "blackout" in Wednesday's 2-1 defeat by promoted Wolverhampton Wanderers where Chelsea squandered the lead in a poor second-half show.
"In some matches we lack determination, like in the last match," Italian Sarri said ahead of Saturday's league game at home to leaders Manchester City.
"Rather than 11 players doing 11 different things, we need a collective target which can give us motivation, and that can give us determination.
"We managed the match against Wolves well for 50 minutes, but we had a blackout when we faced difficulties... maybe we were able to win with some problems in the early part of the season, but now we have to face difficulties."
Sarri, who replaced Antonio Conte and changed Chelsea's counter-attacking approach to a quick-passing, possession-based style, said he expected the team to struggle at some point.
"I thought that would happen in the first two months but we played well and gained points, so I was sure the problems would arise in November or December," he said.
"You need to change the mentality in 25 minds to change the way you play football. And you cannot do that in one day. It is very difficult... it is a long way to go."
The former Napoli boss was asked about the secret to defeating Pep Guardiola's unbeaten City.
"I don't know. I have lost every match against Guardiola so you have to ask somebody else," Sarri joked.
"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 and improve. We know very well it is very difficult."
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Toby Davis)
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