United have been in fine fettle since appointing Ole Gunnar Solskjaer as interim manager and he is in the frame to take over in the summer permanently.
Ed Woodward is set for a massive spending spree in the upcoming transfer window to bankroll the manager.
A number of signings have been linked, including big names such as Gareth Bale, Kalidou Koulibaly and Antoine Griezmann.
One position that Solskjaer wants to strengthen is his midfield – in particular a No.10, with Juan Mata looking likely to leave.
Benfica whizkid Felix has been hotly touted to become one of the best in the world, despite being just 19.
And the Portuguese starlet has already been linked with a number of big clubs – with United being one of them.
But Benfica chief Vieira has warned off the Red Devils by stating that they plan to raise the release clause in his contract.
“I am happy that all of Europe is talking about Benfica’s talents,” Vieira told Tuttosport.
“Now it is clear that our project of betting on young players is bearing the desired results.
“Joao Felix the next Ronaldo? There is only one sure thing: Felix is one of the best talents produced by Portuguese footballer since Ronaldo. Juventus’ and Real Madrid’s interest in him? Ask them.
“At this moment we are not interested in selling him and soon we will increase the release clause in his contract.
“Joao has an incredible technique, he is fast and also very intelligent.
“He is a good person, he shows that he has important values that make me very proud.”
United have been linked with Felix’s team-mate Ruben Dias as well.
The centre-back is another star touted for a big future and has also courted attention from Arsenal and Bayern Munich.