The 19-year-old, who played for England at both Under-16 and Under-17 level, moves back to the Eredivisie club where he spent the second half of last season on loan.
Ali, who was born in Somalia, came through the Chelsea academy and was part of the team which won the FA Youth Cup and UEFA Youth League in 2015/16.
A statement on Chelsea's official website added: " We thank Mukhtar for his service and wish him well for the future."
Source : PA
Source: PA