The 32-year-old, whose current deal is set to expire at the end of the season, has struggled with form and fitness this season - making just 13 Premier League appearances, losing five of them.
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"At the moment, I would rule out an extension with Manchester United," he told Radio Crc.
"Vidic, given his contract situation, has several teams looking at him. Vidic is a very good player and will have no trouble finding a new club. For the time being we are remaining calm."
When questioned on speculation surrounding Napoli Martina denied any concrete interest, adding: "At the moment there has been no contact."
"Thanks to the management of [President Aurelio] De Laurentiis and [sporting director Riccardo] Bigon, they are one of the best clubs in Europe."