Portuguese publication A Bola claims a United representative watched the 22-year-old in action in Braga’s 3-3 draw with Vitoria SC.
The young winger has also been linked with the likes of West Ham, Porto, Barcelona, Atletico Madrid and Monaco.
Silva has a £15.5million release clause in his contract which expires in 2019.
He revealed earlier this month that he turned down offers in January: “I’ve received some offers but I preferred to stay here. My goal is to give everything for this club.”