Benfica coach Jose Mourinho says Gianluca Prestianni's career under him will be "over" if he is guilty of racially abusing Real Madrid's Vinicius Jr.
Prestianni, 20, was accused by Vinicius during the first leg of their Champions League knockout phase play-off tie in Lisbon on 17 February.
The Argentine, who denies racially abusing Vinicius, missed the second leg after being given a provisional one-match ban by Uefa pending the result of a full investigation by an ethics and disciplinary inspector.
He could be punished further once that is complete.
Mourinho was criticised for saying after the match he believed Vinicius had incited the crowd by dancing at the corner flag after scoring the only goal of the game.
Speaking on Sunday, Mourinho said he was "completely, utterly opposed to any kind of discrimination or prejudice, or ignorance, or stupidity".
"If my player did not respect these principles, which are mine and Benfica's as well, then that player's career with a coach named Jose Mourinho and at a club named Benfica will come to an end," the 63-year-old said.
"I am not a scholar, but I am not ignorant either. The presumption of innocence is a human right, isn't it?
"I stand by my opinion. If the player is indeed guilty, I will never look at him the way I looked at him before, and with me, it's over.
"But I have to put many 'ifs' in front of it."
Benfica have "categorically" denied Portuguese media reports that Prestianni admitted to team-mates and club management he had "uttered a racist insult".
In a statement, the club said: "As has already been made public, the player apologised to his team-mates for the incident that occurred during the match against Real Madrid, regretting its magnitude and consequences and assuring everyone, as he has done from the very beginning, that he is not racist."
During the controversial game at the Estadio da Luz, Mourinho was sent off for complaining that referee Francois Letexier was intentionally avoiding booking Real players.
Mourinho, who managed Real from 2010-13, later told journalists "a stadium where Vinicius plays, something happens, always".
"I love Alvaro [Arbeloa, the Real Madrid manager] and I will continue to love him, but I still think that I was the one that took the correct position and not him," Mourinho added.
"I wanted to be fair, neither defending my player nor attacking the other."
Vinicius scored again in the tie's second leg in Madrid, netting Real's second goal as the Spanish side came from 1-0 down on the night to win 2-1 and book their place in the next round with a 3-1 aggregate victory.
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