The 24-year-old midfielder has been with Wolves for the past four seasons, but he is free to leave the Molineux for £35m this summer.
According to Eurosport, the Red Devils are making inroads towards a potential transfer for the Portugal international.
The 20-time English champions need to recoup funds from player sales first. Paul Pogba and Jesse Lingard are likely to depart in the coming weeks.
Pogba is currently on the radar of Paris Saint-Germain, who are willing to pay £43m.
Lingard may drop down the pecking order despite a successful loan spell at West Ham United this year.
The Hammers are interested to re-sign the England international, who is valued at £25m.
Man United are also keen on signing Atletico Madrid right back Kieran Trippier this summer.