The 32-year-old was a regular starter under Antonio Conte last season, but new manager Maurizio Sarri is yet to include him in any of his Premier League squads this term, while he was an unused substitute during Thursday night's Europa League win over PAOK.
Sarri has previously suggested that Cahill does have a future at the club, but the England international admitted that he is finding it "difficult" to adapt to being a bit-part player.
"I don't want to jump the gun but as things stand at this time, probably, yes [I might have to leave in January]," Cahill told The Mirror.
"First of all the team's doing well which is the most important thing. Having been a big part of things for six years, seven years and played a huge part, not just a squad player, I'm definitely finding it difficult this year to deal with that situation.
"To go under a new manager, learn a new way of defending and new ideas that he has I am sure he didn't feel comfortable in putting me in. So there's no arguments from my point of view, hence why I've not seen him. When you're winning games you have to sit tight and suck it up and deal with it. For me to do this all season is going to be very difficult."
Cahill joined Chelsea in 2012 and has gone on to make 282 appearances for the club, helping them to six major trophies including the Champions League and two Premier League titles.