However, he insists that the Argentine's decision to pull up his shirt to cover his mouth does suggest it was "something offensive".
Vini Jr struck for the 1-0 Champions League playoff first-leg win, though his celebrations angered home fans. He was booked for his behaviour before he clashed with Prestianni as they walked back to the centre circle.
The Brazilian reported what was said to the referee, accusing the Argentine of alleged racial abuse. The match was then suspended for 10 minutes as per UEFA protocols before it resumed as controversy swirled.
Valverde later remarked: "Nobody knows exactly what was said to him, but according to all the teammates who were nearby, something offensive or inappropriate was said. We have been fighting against this for a long time. It's unfortunate that a camera didn’t capture it.
"I think if you cover your mouth to say something, it’s because you’re saying something that’s not right. It’s lamentable. I’m proud of the teammates who defended Vini, who kept moving forward and played a spectacular match."
On leaving the pitch, Valverde also stated: "It’s something very serious. It has happened many times already. At this point, it could be an option, even if it doesn’t sit well with the spectacle of football. But there’s also a person who has damaged the spectacle by saying something inappropriate to Vini Jr.
"It goes against everything we are working towards. We, as his teammates, will help and support him. Throughout the match, all kinds of things were said. I doubt there isn’t a camera that didn’t capture that."
On winning the first-leg, Valverde added: "We have to focus on the team performance, the match that Vini Jr. and Kylian played. They helped us a lot defensively, which was really needed today. They made spectacular plays up front, and I think we’re leaving with a good result.
"If we all run a little to press, recover the ball as quickly as possible, and stay united when we have to defend in our half, it becomes much easier. You feel much more energized when attacking, you’re much closer to the opponent's goal, and I think that showed today."