The 33-year-old Brazil international joined QPR from Internazionale last summer and has made 26 appearances this season.
However, he has been unable to prevent the club from slipping to the verge of relegation from the Premier League.
Cesar was pictured wearing the home shirt of QPR's London rivals as he posed with countryman David Luiz at a birthday party for the Blues defender.
The image was quickly circulated on social media platforms, with QPR head of communications Ian Taylor writing later on Twitter: "The club is aware of an image of Julio Cesar circulating on social networking feeds.
"The image, which appeared on an Instagram account, has since been deleted. The club will be dealing with this issue internally."
Cesar later issued his own statement through his club, saying: "I would like to clarify that last night I attended David Luiz's fancy dress birthday party and as a joke for my friend, my family and I dressed as him.
"I want to assure everyone that absolutely no offence was intended."