Barcelona are believed to be interested in signing the starlet next season, although Benfica are obviously keen to put off potential suitors with such a high price.
Next season a clause in Mantorras's contract is believed to become activated, which would give Barca first option to sign the precocious teenager, although Vieira warned it would cost the Catalan side dearly.
Vieira told A Bola: "If Barcelona exerts its right, I only can say that the amount that would have to be paid is high. It would be one of the major transfers of worldwide soccer.
"The cost of Mantorras would be superior to the one which Madrid paid for the transfer of Figo."
Vieira denied that the forward had already been sold for next season, but admitted that 'Barcelona has a correct, preferred option' on Mantorras, which was negotiated when Simao (pictured) was sold to the Portuguese side for 2,000 million pesetas (£7.2 million).
According to Vieira, who coached Mantorras at Alverca last season, Benfica paid £3 million for 50 percent of the player's rights.
Vieira also did not confirm whether or not Benfica had already sold on Simao to another European side, saying only that: "The future will demonstrate it was excellent business."
