32-year-old Mertens has enjoyed a successful spell at Italian outfit SSC Napoli but will reportedly leave the club at the end of the current season after the club refused to hand him a two-year deal worth €6m per year demanded by him.
Inter Milan have been tipped to sign the Belgian playmaker at the end of the season. But, according to TuttoMercatoWeb , the Red Devils and Borussia Dortmund are set to provide tough competition for his signature.
The report further claims that Manchester United have already started talking to Mertens' representatives with the player free to talk to other clubs in January and sign a pre-contract agreement.
However, Inter remain the favourites to land Mertens, with Antonio Conte keen to bolster his attacking options and link up Romelu Lukaku with his Belgium team-mate.
Reports also claim that Mertens himself is keen to stay in Italy but United are reported to be trying to convince him to make the move to England.
Additionally, apart from Mertens, Manchester United have also sent scouts to Italy to watch teen sensation Dejan Kulusevski, with the Swedish midfielder impressing on loan at Parma from Atalanta.