Januzaj, 21, has seen his career stagnate in the past two seasons after bursting onto the scene under David Moyes in the 2013-14 season.
He was loaned out to Borussia Dortmund for the first half of last season but the deal was terminated after he made just three starts for the club, and the second half of last season saw him on the fringes at Old Trafford.
The Belgium international has another chance to resurrect his career under Moyes at Sunderland, and scored the winner in the club's EFL Cup win over Shrewsbury Town.