The 32-year-old center back is out of contract at the end of the season and is yet to play under new coach Maurizio Sarri, who has led Chelsea to the top of the Premier League table with five wins from as many matches.
"I don't want to jump the gun, but as things stand at this time, probably, yes (I'll have to leave in January)," Cahill said. "For me to do this all season is going to be very difficult.
"Obviously the most important thing is the club. But I have to look at my situation. I have to take charge of my own future. And if I'm sat doing what I'm doing at this moment in time, I just hope the club will respect that when it comes to the time."
Cahill, who won the Champions League with Chelsea in 2012 and the Premier League in 2015 and 2017, retired from England duty last month.
"I'm not a guy who is willing to accept not playing," he said. "Your career's short enough as it is."
Chelsea visit West Ham on Sunday.