The Belgian has become a star for Real Sociedad in recent seasons.
And Spanish outlet Don Balonreports United are weighing up whether to trigger a clause to bring Januzaj back to Old Trafford.
United would be able to sign Januzaj, who is under contract until 2022, for a fee of €35m.
It is significant increase from the €8m they recouped from selling Januzaj to the San Sebastian outfit in 2017.
He made 63 appearances across all competitions before departing.