Okon was named on the reserves' team sheet for Wednesday night's game against Sunderland at Belle Vue but he jetted off to speak to his proposed new club.
He dropped out during the day and is believed to be talking to Italian Serie B club Vicenza.They see him as one of the players who can help their bid for promotion this season.
The 31-year-old played for Fiorentina and Lazio during his spell in Italy before moving to Middlesbrough.
Frank Farina, Australia's coach, said before the Ireland friendly that Okon had to be playing first-team football to stay in the national side.
Okon has made only 21 appearances for The Whites since joining them at the start of last season.