West Ham are adamant that Diop is not for sale and it is believed they would not even listen to offers of less than £60m.
Diop played 38 times last season after the Hammers signed him from Toulouse a year ago for £22m.
United’s top priority is signing a centre-back this summer and they have been told by Napoli that Kalidou Koulibaly would cost about £100m while Leicester value Harry Maguire at £80m.
Former United manager Jose Mourinho singled out Diop for praise when his side lost 3-1 at West Ham in September last year.
As well as a centre-half, United are intent on bringing a new right-back to Old Trafford to replace departing club captain Antonio Valencia and are preparing a second bid for Crystal Palace’s Aaron Wan-Bissaka after their first £40m offer was rejected.