The Belgian star only signed a new contract in May but is still being watched by European clubs after a fantastic campaign last season, where he scored 28 goals.

And earlier this week Napoli's former director Pierpaolo Marino admitted the former PSV man has a £25m release clause in his deal.

"The €28m clause is likely to be a low price even if he is 30," Marino told Italian newspaper Tuttosport.

"It turns out that United are following him very closely."

Napoli have won each of their seven games in the league this season.

And Mertens has been instrumental in their success, scoring seven and assisting twice for the Serie A leaders.

The 30-year-old's most impressive moment of the season came against Lazio, where he audaciously lobbed the goalkeeper with an incredible piece of skill.

Jose Mourinho's men have been in fantastic form this season and currently sit joint-top of the Premier League with Manchester City.

However, the Portuguese boss attempted to strengthen his side with wide players in the summer but missed out on Inter Milan's Ivan Perisic, meaning he may go back in the market for a winger in January.

Mertens has been linked with a move to England in the past.

"Yes there was interest [from Chelsea], and from other clubs also," Mertens said over the summer.

"But in my head there was never an intention to change clubs.

"This season was really good. I am gong to try and be even better next year."

Manchester United take on Liverpool after the international break.