Mastour started his senior career at San Siro Stadium outfit in 2014 after a move from AC Reggiana.
In order to get first team opportunities, the Morocco international was loaned to Málaga for two years, making only one substitute appearance before the deal was terminated in 2015.
He then joined PEC Zwolle on a season-long loan deal, also making only one appearance for the club.
The 20-year-old who had poor performance in the 2015-16 league campaign will be hoping to reignite his career in Greece
The youngster made his international debut for the Atlas Lions in a 2017 Africa Cup of Nations qualifier against Libya in 2015.