Fountas was accused of using the N-word after tangling with Inter Miami defender Damion Lowe (29) during the second half of Sunday's clash at Audi Field.

Greece international Fountas was substituted by United boss Wayne Rooney after Miami players said they would not continue to play if he remained on the field.

However on Monday Fountas strongly denied using the language which halted play for several minutes.

"I did not use the word I am being accused of using," Fountas said in a statement on Instagram. "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 always respect the culture, religion, and skin color of each person. I am therefore very upset by this accusation and saddened to be falsely accused."

Major League Soccer has opened an investigation into the incident, warning it had "zero tolerance" for abusive language.

"MLS has zero tolerance for abusive and offensive language and we take these allegations very seriously," the league said.

"An investigation into this matter will begin promptly. Further information will be provided upon completion of that investigation."

Inter Miami boss Phil Neville said after the game that Fountas had used the "worst word in the world."

"We're a club that is proud of our diversity and inclusion in our staff and supporters, and there is no place at all for racism on a football field or society," Neville said.

"A word was used that is unacceptable in society. It is the worst word in the world."

Miami's US international defender DeAndre Yedlin said he expected MLS to take action, describing the incident as an "incredibly dark moment."

"We'll see what MLS does about it," Yedlin said. "My eyes will be on that a lot."

"It's up to them to take action and make a stand and show that it not only has no place in the game but no place in society," he added.