Benfica officials are fearful Newcastle could trigger Enzo Fernandez's £105 million release clause, with the Premier League side already seeing a bid rejected, according to reports in Portugal.
Interest in the 21-year-old has risen following his performances at the World Cup in Qatar, with Fernandez rewarded for his role in Argentina's triumph by being named Young Player of the Tournament.
Benfica President Rui Costa has turned down an initial offer from Newcastle for the midfielder, according to Portuguese newspaper A Bola.
The Portuguese side were reported to have rebuffed a £88million offer for midfielder last week in the hope a club would pay the full release clause.