Andy Robertson was overheard telling Liverpool team-mate Sadio Mane he'd get revenge on Flamengo's Rafinha after he got the forward booked.
Mane was adjudged to have fouled the 34-year-old on the stroke of half time, and was visibly unhappy at being shown a yellow card in the Club World Cup final.
Robertson walked behind Rafinha seconds later and gestured something to Mane, in ear-shot of the Brazilian.
Replays showed that the Scottish left-back uttered: "Sadio, I'll get him don't worry" - making sure Rafinha got the picture.
Liverpool had to work hard to become Club World Cup champions, with Roberto Firmino netting the game's only goal in the first half of extra time after a bore draw.
Liverpool captain Jordan Henderson said: "It was an interesting game. I am sure it was interesting to watch but we kept going, good performance and mentality shined through.
"We could have scored a couple more. Overall delighted with the result and performance in difficult conditions.
"We have found a way for a long time now. Some late goals, we just want to keep going, keep improving. Keep working hard and putting in performances like that."
Speaking on BBC One, former Liverpool midfielder Danny Murphy added: "They deserved it.
"They were the better team and they had the better legs as the game went on and it was a great goal. Liverpool continually get results not always playing the free-flowing football we expect.
"It was the trophy Liverpool was missing. They have been a successful club for many years and now they've won the lot."
