The Blues sit third on the table, separated from the Citizens by Manchester United, who have three more points than Antonio Conte's men.
"It's make it or break it - December, it's always very difficult," the Ghana international told ESPN FC.
"After December, you can tell who's going to win the title. Chelsea have a big squad that can stand up to every game, so I hope they can go through December in a good way.
"You can't say the title race is over just yet, but 11 points is quite a lot especially with Man City being very dangerous at the moment.
''It seems like no one can stop them, but I still think someone can. It's up to Chelsea to win their games and keep up with City, and hopefully Chelsea don't slip up."
Essien, currently on the books of Indonesian side Persib Bandung, played for Chelsea between 2005 and 2014.
During that period, the 34-year-old won the English Premier League on two occasions (2005-2006 and 2009-10), his trophy haul also including four FA Cup titles, one League Cup, two Community Shields and the prestigious Uefa Champions League (2012).