The treble winners cruised to a 4-0 victory over the Brazilian giants in Saudi Arabia before a fight broke out at full-time.
Veteran defender Felipe Melo was seen scrapping with City captain Kyle Walker with Grealish stood nearby.
Speaking afterwards, Melo claimed he went for Grealish because the England star was 'disrespectful' to Fluminense star Matheus Martinelli.
Melo said: "When the game ended, Grealish went after Martinelli. He chose a little guy. He has no balls to face a big one.
"Grealish was passing close to Martinelli and I saw them talking, forehead to forehead, so I went after him. I pushed him. Walker pushed me, too. I went to defend a teammate."
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The 40-year-old added: "I defended my institution. I will do this forever. No one will disrespect my institution or its athletes. No one will come and humiliate my friends or my club.
"I didn't want a mess. Grealish was disrespectful towards the Fluminense institution and I will never let that happen.
"He was shouting 'ole' at the end of the game. This is something that the fans can do, not a player - an athlete on the field cannot be disrespectful.
"I went to defend Martinelli, who was being cornered by this athlete. And I would do it again. I am a warrior. Nobody can ever crow with a rooster."
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But England forward Grealish was not having it, responding to Melo's quotes on X, formerly known as Twitter.
The 28-year-old said: "Not once did I say 'ole'."
However, midfielder Martinelli offered a similarly damning version of events to Melo.
The 22-year-old said: "I didn't understand what he said, everything was in English.
"I wanted him to be sent to hell because he was talking aggressively to me.
"I did not talk to him at all, so why he had to come and tease me?
"That's football, the guy is really also disrespectful during the game. But it's football, life that goes on. He's just a bad sportsman."
And they were not the only Fluminense stars to castigate Grealish for his alleged behaviour on the pitch.
Real Madrid legend Marcelo said: "Everyone does what they want when playing.
"All I will say about Grealish is that he has won this competition once - and I have won it four times. I would never act like that."
Colombian forward Jhon Arias added: "Grealish was very bad. After the fourth goal he started saying 'ole' and making fun of us.
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"This is simply not sportsmanship. Sometimes you win and sometimes you lose, but you have to be a man at all times and he lacked a man's behaviour.
"You can't do that after scoring a fourth goal. It's no surprise it didn't sit well with Felipe Melo - you know what his temper is like."