Mertens has developed into one of the most potent attackers players in Europe after scoring 28 goals in 35 appearances last season.
And last night, the 30-year-old Belgian scored one of the goals of the season when he produced a stunning lob from a tight angle in the 4-1 win over Lazio.
Calciomercato.com says the striker has a £26.4million (30m euros) buyout clause which would be affordable for most top clubs in Europe.
The report says United and Liverpool 'can't believe' how low the clause, which can be triggered next summer and is only valid to clubs outside of Italy, is.
It is claimed United held talks with Mertens before the end of last season but were unable to reach an agreement with Napoli.