So then, Jose Mourinho and Antonio Conte have engaged in a war of words that doesn't like de-escalating anytime soon.
Just how it got all started? Check out this timeline.
OCT 2016 -- United lose 4-0 to Mourinho's former club and he accuses Chelsea boss Conte of 'humiliating' him with his celebrations.
MARCH 2017 - The pair square up on the touchline at Stamford Bridge as Chelsea knock 10-man United out of the FA Cup with a 1-0 win.
JULY 2017 - Conte says new champions Chelsea 'want to avoid the Mourinho season', in reference to how they collapsed after their 2015 title win.
OCT 2017 - Mourinho says other managers 'cry' over injuries. Conte says: 'He has to think about his team and start looking at himself, not others.'
NOV 2017 - Chelsea beat United 1-0 and there is no handshake at full time.
JAN 2018 - After exchanges in press conferences, Mourinho says: 'What never happened to me, and will never happen to me, is to be suspended for match-fixing.' Conte hits back by labelling Mourinho a 'little man'.