The France Under-21 international starred for the Hammers in his debut season in England, making 38 appearances in all competitions.
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And the centre-half’s displays have caught the eye of United boss Ole Gunnar Solskjaer.
Sky Sports claims the Red Devils are willing to pay £45million and offer a player for Diop, but West Ham will not entertain bids of less than £60million.
Diop joined West Ham in a £22million deal from Toulouse last summer and has four years left on his contract.
His performance in the Hammers’ 3-1 win over United last September led to praise from former Old Trafford boss Jose Mourinho.
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The Portuguese coach said: “He had a fantastic match.
“Congratulations to the scout that found a 21-year-old kid Diop – a monster who dominated in the duels.”