Arsenal scout Gilles Grimandi and Manchester United's Marcel Bout were in the stands to watch Monaco against Manchester City on Wednesday.
Monaco's Kylian Mbappe and Benjamin Mendy, who were called up to the France squad yesterday, are under consideration.
United have also watched Tiemoue Bakayoko and Fabinho closely for the past year.
Bakayoko is seen as a potential foil for Paul Pogba in central midfield.