The Premier League clash was hotly-contested on the pitch at Old Trafford and tempers flared as the teams headed back down the tunnel just moments after the final whistle.
According to The Daily Mail, City’s goalkeeper Ederson confronted Mourinho and the pair exchanged words before a melee involving more than 20 players and staff broke out.
It’s claimed Mourinho was squirted with water and was also struck on the head by a plastic bottle during the brawl.
A number of players were seen swinging punches at each others while police, who were already present in the tunnel, watched on in shock.
‘Nobody minds celebrating but the lads deemed that excessive,’ a United insider told The Daily Mail.
Asked about United’s defeat, Mourinho told Sky Sports: ‘It’s a clear penalty. I’m sorry for us and I’m sorry for Michael [Oliver] – he had a good performance, but it’s a clear penalty.
‘The referee made a mistake which can happen. Last season it was similar when we were winning 2-1.
‘The referee is a human being he tried his best, he’s committed to having a good match which I think he had.
‘He made one mistake which is crucial for result.’
Manchester United 2017/18 Fixtures
West Bromwich Albion (A) – Sun Dec 17
Bristol City (A) – Wed Dec 20 (EFL Cup)
Leicester City (A) – Sat Dec 23
Burnley (H) – Tues Dec 26
Southampton (H) – Sat Dec 30
Everton (A) – Mon Jan 1
Stoke City (H) – Sat Jan 20
Tottenham (A) – Wed Jan 31