Vinicius Jr said he was racially abused by the Benfica and Argentina player during a Champions League match in February.
The Madrid star left the field and refused to return after Prestianni's comments, resulting in a stoppage in play that lasted 10 minutes.
Prestianni denied the racist allegations after the match on his social media, saying Vinicius Jr “regrettably misunderstood what he thought he heard”.
Benfica boss Jose Mourinho said Prestianni's career under him would be "over" if the abuse was proven after UEFA’s investigation.
It was reported in February that Prestianni had told UEFA he had used a homophobic slur rather than a racist one which has now led to his six-match ban by UEFA.
As per ESPN, the ban also includes the one match Prestianni served as a provisional suspension in February, so unless he triggers the suspended three-match ban, he will actually be sidelined for only two further games.
The ban also includes a request to Fifa to extend worldwide the above-mentioned suspension which means the punishment would apply to matches at this summer’s World Cup.
Fifa president Gianni Infantino said players who cover their mouths when speaking to opponents during confrontations should be sent off and this matter could be discussed at the FIFA Congress next week.