Júlio César has apologised to Queens Park Rangers supporters for wearing a Chelsea kit to David Luíz's birthday party.
Goalkeeper César, dropped in favour of Robert Green for the weekend defeat at home to Stoke City, claimed he meant "no offence" but QPR said the matter would be dealt with internally.
“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," César said.
“I want to assure everyone that absolutely no offence was intended.”
The pictures of César appeared on Luíz's Instagram account but have since been deleted.
César, 33, is likely to be fined by the Rs, who have said they will not comment any further on the matter.
The Brazil international was one of 11 players to arrive at Loftus Road in the summer under Mark Hughes, however QPR are destined for relegation after a fourth defeat in five.
Local rivals, the enmity between QPR and Chelsea intensified last year when Chelsea captain John Terry was accused of racially abusing Anton Ferdinand. The former England skipper was suspended for four matches by the Football Association.