The winger has been a revelation for RBL this season, with his form attracting the interest of Europe's biggest clubs.
Sky Deutschland says United, Bayern Munich and PSG are all interested in Diomande.
At Old Trafford, the young Ivorian has been identified as the first alternative after the failure to sign Antoine Semenyo, of Bournemouth, who is now close to joining Manchester City.
Manager Rúben Amorim sees Diomande as an ideal candidate for a gradual development project, rather than a quick fix.
The arrival of Bayern and PSG, however, changes the scenario: the Bavarians consider him a strategic investment for the future, while the Parisian club is convinced that his qualities can make the difference both in Ligue 1 and the Champions League.