The Mancunian giants had supposedly prepared an offer worth £45 million for the centre-back, which also included an unnamed player as a part of the deal.
This bid from the Old Trafford club has now been confirmed in France, according to TF1 journalist Julien Maynard.
As per his update, West Ham have turned down the tabled offer, as they intend to hold on to the 22-year-old for the upcoming campaign.
Diop made 38 appearances for the Hammers in all competitions last season, and manager Manuel Pellegrini considers him a valuable part of his squad.
However, Maynard adds that Diop himself prefers to secure the move to the Theatre of Dreams, despite the resistance from his club.
It will be interesting to see whether the Red Devils return for the former Toulouse starlet with an improved bid, or offer another player as a part of the exchange deal.