According to Guardian journalist Daniel Taylor, bottles were flying as well as fists following the contest, and United striker Romelu Lukaku was 'prominently involved'.
This bust-up in the tunnel at Old Trafford today. It wasn't handbags: Mikel Arteta was left with blood pouring down face. Bottles as well as punches thrown. Romelu Lukaku prominently involved, according to witnesses.— Daniel Taylor (@DTguardian) December 10, 2017
The Sun quotes an eyewitness source as saying, "People couldn't believe what they were seeing. It was going crazy for a few seconds, with loads of screaming and shouting. It looked like it might get completely out of control."
Mourinho is said to have taken exception to City's loud celebrations in the away dressing room and apparently stormed in and confronted goalkeeper Ederson, whom the United boss accused of wasting time in the closing stages of the game.
He is said to have shouted 'Show some respect!' at the goalkeeper, garnering a strong response from the other City players in defence of their team-mate.
In a scene reminiscent of the infamous 'Battle of the Buffet' when Sir Alex Ferguson was hit by flying pizza thrown by Arsenal player Cesc Fabregas, BBC Sport reports that Mourinho had water and milk thrown at him during the post-game exchanges.
It had been a frustrating day for United. Jose Mourinho's side had seen their 40-game unbeaten home record come to an end, while City won a Premier League record 14th straight game to extend their lead at the top of the table to 11 points.
"Manchester City are a very good team and they are protected by the luck, and the gods of football are behind them," Mourinho commented.