Major League Soccer said it will look into a claim that DC United player Taxi Fountas used a racial slur against Inter Miami defender Damion Lowe during the second half of Sunday's match.
Inter Miami won the game 3-2 to give their play-off hopes a significant boost, with Gonzalo Higuain scoring the deciding goal.
But the mood was sombre afterwards with Inter Miami players revealing they were ready to walk off the pitch during the match if something was not done about what they believe Fountas said.
"It was a racist comment," Inter Miami coach Phil Neville said. "It was unacceptable. A word was used. I think it's unacceptable in society.
"A word was used, I think, the worst word in the world. And that's it, really."
The DC United player has denied using that word, issuing the following statement: "I did not use the word I am being accused of using. That despicable racial slur is one I denounce and did not use.
"We had a hot discussion on the field, but I have not racially abused anyone. I firmly reject racism in any form, it is despicable.
"I have many friends from many cultures. I always respect the culture, religion and skin colour of each person. I am therefore very upset by this accusation and saddened to be falsely accused."
Fountas and Lowe both received yellow cards in the 62nd minute after exchanging words. That all happened shortly after Fountas scored to tie the game at 2-2.
Inter Miami players gathered and after a period of conversation that lasted several minutes, Neville spoke with Wayne Rooney, his DC United counterpart. Rooney took Fountas out of the game, and play resumed.
"There was a complaint, which I'm sure will get investigated. Not much more I can say," Rooney told reporters.
Inter Miami's DeAndre Yedlin said: "If nothing was done, we weren't going to continue.
"At the end of the day, it doesn't even feel like a win because of that incredibly dark moment.
"So now, we'll see what MLS does about it. My eyes will be on that a lot, to see what kind of action they take, because it's now up to them to take action and make a stand and show this not only has no place in the game but no place in society."
After the match, MLS released a statement to confirm an investigation into the matter will begin promptly.
"MLS has zero tolerance for abusive and offensive language, and we take these allegations very seriously," the league said.