Former Napoli sporting director Pierpaolo Marino has claimed that Manchester United are "following" Dries Mertens "very closely".
Mertens only recently signed a new long-term contract at Napoli, but the deal includes a release clause of £25m.
The 30-year-old has scored 43 times in just 56 appearances for his Italian club since the start of last season, and according to Marino, United boss Jose Mourinho is considering an approach.
"The €28m (£25m) clause is likely to be a low price even if he is 30," Marino told Calcio Mercato. "It turns out that United are following him very closely."
Mertens joined Napoli from PSV Eindhoven in the summer of 2013, and enjoyed his best scoring season last term - hitting 34 goals in 45 appearances in all competitions.
