The Sun says the pair were shouting so loud at their team-mates that the furious tirade was heard in the tunnel by members of City's backroom staff.
Erik ten Hag stood by and watched as his players took matters into their own hands in the dressing room. He agreed with Fernandes and Martinez and let them criticise his squad.
It looked as though United's half-time team talk had worked when Antony pulled one back with a superb strike from distance.
But any hope of a comeback was quickly extinguished when Erling Haaland and Phil Foden made it 6-1 with both men securing their hat-tricks.
United did get two more goals from substitute Anthony Martial, but only after it appeared that City had taken their foot off the gas by making four changes after the 75th minute.