Benfica have begun an investigation into two fans who were filmed making what appeared to be monkey gestures in the direction of Real Madrid's Vinicius Junior during the two sides' Champions League match earlier this week.
During Tuesday's match in Lisbon, Brazilian forward Vinicius accused Benfica's Argentine midfielder Gianluca Prestianni of racially abusing him.
Prestianni, 20, denies calling Vinicius a monkey and Benfica have stood by their player, claiming he was the victim of defamation.
European football's governing body UEFA has said it will "investigate allegations of discriminatory behaviour".
Portuguese media on Friday claimed that Benfica had begun their own investigation into the fans' behaviour.
"If they are club members, the procedure could lead to their expulsion," a Benfica spokesperson told AFP.
In a video shared on social media, including by former England international Rio Ferdinand, two fans wearing Benfica's red jersey can be seen waving their arms in a way typical of imitating a monkey.
Real won the Champions League play-off first leg 1-0 thanks to Vinicius's wonder strike.
The original clash between him and Prestianni was sparked by Vinicius's goal celebration, which drew criticism from Benfica coach Jose Mourinho.
Vinicius and Prestianni, who had covered his mouth with his jersey, appeared to be arguing for some time before the Brazilian suddenly ran over to French referee Francois Letexier to lodge his complaint.
Real said on Thursday they had submitted evidence to UEFA to support Vinicius's allegation.