Manchester United and Manchester City have been asked for observations by the FA after a tunnel altercation at Old Trafford on 10 December.
Jose Mourinho was annoyed by the loud music and celebrations from the Manchester City’s open dressing room.
Mourinho was making his way to his post-match press conference following their 2-1 defeat. It was then when an altercation followed when he made his feelings known to City goalkeeper Ederson.
The United boss then had a carton of milk thrown towards him from inside the dressing room which reportedly hit one of his backroom staff.
Manchester City assistant coach Mikel Arteta suffered a cut to the head in the incident but no punches were thrown.
Pep Guardiola was encouraged to get involved with Manchester City’s post-match celebrations but declined.
It is understood Manchester United were surprised at the manner of Manchester City’s celebrations after the full-time whistle.
The incident was not included in referee Michael Oliver’s match report, so the FA will now investigate.
The clubs have until 13 December to respond.
Manchester City posted images of players celebrating in the tunnel following their 2-1 victory.
The win moved them 11 points clear at the top of the Premier League.