Manchester United and Manchester City have been asked for observations by the FA after a tunnel altercation at Old Trafford on Sunday.
Jose Mourinho had milk thrown at him in the tunnel area, according to Sky sources, while Manchester City assistant coach Mikel Arteta suffered a cut to the head in the incident.
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 December 13 to respond.
Manchester City posted images of players celebrating in the tunnel following their 2-1 victory, which moved them 11 points clear at the top of the Premier League.