The Football Association has written to Manchester United and Manchester City to seek their observations over the tunnel bust-up at the end of Sunday's clash at Old Trafford.
Referee Michael Oliver did not witness the incident and told the FA on Monday morning that he did not submit an incident report because he was not aware of the clash at the time.
As a result, the FA has given both clubs until 6 p.m. GMT on Wednesday, Dec. 13, to respond. The FA can seek CCTV footage from the tunnel area should they opt to pursue an investigation.
Around 15 players and coaches were reportedly involved in a melee near the dressing rooms after City's 2-1 win. Sources said United believed City's celebrations were "disrespectful" and "very vocal."
A source close to City said the incident started when United boss Jose Mourinho entered the visitors' dressing room and that while loud music was being played, it was nothing out of the ordinary.
The City source also confirmed that their assistant coach Mikel Arteta had a cut to his face but refuted reports that said Arteta was "pouring with blood" and that milk or water was thrown at Mourinho.
United could also find themselves in hot water with the FA after a smoke bomb appeared to be thrown from the stands towards Bernardo Silva in the closing stages of the game.
The object did not hit the City midfielder, landing a few feet away before being moved to the side of the pitch. The FA have confirmed they are investigating the incident and have written to United to ask for their observations.
City do not plan to make any formal complaint, according to the source, and view the matter as "argy-bargy."
There were police officers present in the main tunnel area, although not in the corridor next to the changing rooms. No complaints have been made.
ESPN FC's Rob Dawson contributed to this report.