In recent days there has been mixed speculation regarding United's supposed pursuit of the Chilean international, with some reports having claimed that a deal was nearing a conclusion, while others suggested that no contact had been made between the two clubs.
Yesterday the rumours intensified further when Juventus head coach Antonio Conte resigned from his position following what many believed a dispute with the board over the club's transfer policy.
However, at today's unveiling of new boss Massimiliano Allegri, Marotta claimed that the Old Lady had no intention of selling Vidal, who scored 18 goals last season.
"Vidal is not for sale, he is one of the most important players in the squad. [Vidal] is happy to be here," he told reporters.
Vidal is currently on holiday in his native Chile following his exploits at the World Cup.