The Belgium international has become integral for the Serie A side since Gonzalo Higuain's exit last summer.
This season Mertens has scored 26 goals to spearhead their push for qualification for next season's Champions League.
Man United's £250m summer wish list Tue, April 11, 2017
Express Sport takes a look at five superstar players Manchester United are reportedly keen on signing
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Romelu Lukaku (Everton) – £65m
Manchester United boss Jose Mourinho is keen to bolster his attacking options in the summer after struggling for goals this term.
United have scored the fewest goals – 46 – among the top six clubs in the league.
Napoli forward Dries Mertens
Zlatan Ibrahimovic, 35, is United's top scorer with 28 in all competitions.
According to Italian outlet Tuttomercatoweb, United kept a close eye on Mertens during Sunday’s 3-0 win over Lazio.
Mertens is said to have a £30million release clause in his contract, which ends in 2018.
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