The Blues currently sit four points clear at the top of the Premier League table without a defeat in their opening eight matches.
"The next two months will decide if this is a genuine title challenge," said Mata. "It's always like that in November and December.
"There are a lot of games and you have to keep fighting for the title in those two months. For me it's the most important part of the season."
Mata scored twice on Saturday as Chelsea came from behind to earn a 4-2 win at London rivals Tottenham Hotspur.