Business Insider Edition
Runaway Premier League leaders City opened an 11-point gap over second-placed United with their 2-1 win on Sunday.
Victory sparked mass celebrations from City. United are understood to have taken exception to how their visitors conducted themselves in and around the dressing room area.
Milk and water were reportedly thrown at United boss Jose Mourinho after a confrontation with City goalkeeper Ederson and this allegedly escalated into altercation involving up to 20 players and staff in a crowded corridor.
City coach Mikel Arteta was reportedly left with a cut below his eye after being hit by a plastic bottle.
The FA had asked for a response from the clubs by Wednesday but they have been given until two extra days to provide observations on the alleged fracas.
"Manchester United and Manchester City have both been given extra time to provide their observations following reports of an incident after Sunday's game. They now have until 6:00 pm on Friday 15 December," the FA said in a statement.
Mourinho has dismissed claims his players were in the wrong during the chaotic post-match confrontation that overshadowed the derby.
City manager Guardiola has also defended his players, although he has offered an apology to United.
