Manchester City defender Bacary Sagna has been asked to explain a posting he made on Instagram which appeared to question the objectivity of referee Lee Mason in Monday's 2-1 win over Burnley.
The Football Association (FA) moved after the 33-year-old Frenchman wrote: "10 against 12.. but still fighting and winning as a team."
Friday deadline to respond
Sagna, who has until 1700 GMT on Friday to respond, later deleted the "10 against 12" part of the posting. He could be charged by the FA at a later date.
City, who could fall 10 points adrift of Chelsea if the Premier League leaders beat Spurs on Wednesday, were not happy with Fernandinho's sending-off in the first half and complained about Burnley's goal because they believed goalkeeper Claudio Bravo had been fouled.
The FA has rules concerning social media and questioning the integrity of officials is strictly forbidden.