The Daily Mail says United will make their move on Tuesday for the 13 year-old.
Nabay is now deregistered as a Blues player.
Given his ties with Birmingham have been severed, the Premier League's elite will be free to make their move for the Sierra Leone-born wonderkid.
And United are due to make their pitch for Nabay to join the Old Trafford academy, with negotiations planned for Tuesday.
However, it is understood Nabay - who scored 12 goals for the club's academy side - has not completely ruled-out re-signing with Birmingham.