It is understood the United boss was reportedly involved in a confrontation with City goalkeeper, Ederson. Figures from United apparently took exception to the raucous manner in which City were celebrating their 2-1 success.
City coach, Mikel Arteta, is understood to have suffered a cut to the head in the fracas, while Mourinho apparently shouted into the City dressing room.
There have also been reports that milk and water were thrown at Mourinho.