The Hammers have allegedly reached an agreement with newly-promoted Fulham to sell Diop for around £13m plus add-ons, with the 25-year-old is expected to undergo his medical in the capital on either Tuesday or Wednesday before finalising his move to Craven Cottage.
Diop was one of four centre-backs who was not involved in West Ham's 2-0 defeat at home to Manchester City in the Premier League on Sunday, with David Moyes forced to play Ben Johnson at the heart of the defence alongside Kurt Zouma.
Moyes has revealed that Diop asked to be left out of the squad for their opening fixture, while he has also confirmed that Craig Dawson, Angelo Ogbonna and summer signing Nayef Aguerd are all recovering from injuries.
Speaking to football.london, Moyes said: "No, Issa Diop is not injured at the minute. Issa Diop didn't put himself forward to play in the last couple of games and we will see what happens with that in the next couple of weeks.
"Difficult when you start the first game against the best team. We've been chasing a centre half for two years, [Aguerd] then injures himself against Rangers, so we have to go with that. [Dawson] has got a bit of a thigh strain, we've not got Angelo Ogbonna ready to get back yet from his cruciate yet, we were really, really short of defensive options."
West Ham are now hoping to recruit at least one centre-back before the September 1 deadline, and the Evening Standard reports that they have identified Paris Saint-Germain's Thilo Kehrer as their primary target.
The 25-year-old, who can operate at either centre-back or right-back, has entered the final year of his contract at the Parc des Princes and is set to fall further down the pecking order following the arrival of Nordi Mukiele and potential addition of Inter Milan's Milan Skriniar.
Since joining PSG from Schalke 04 for around £31m in the summer of 2018, Kehrer has made a total of 128 appearances for the Parisian giants, including 20 starts in 34 games across all competitions last season.
Kehrer's teammate Abdou Diallo is another defender on West Ham's shortlist, according to Sport Witness, with PSG willing to listen to offers for the 26-year-old.
Diallo, who has also been linked with AC Milan, featured only 16 times for PSG last season and the Frenchman is now deemed surplus to requirements by new head coach Christophe Galtier.
Meanwhile, Daily Express journalist Ryan Taylor claims that Fenerbahce centre-back Attila Szalai is another name on West Ham's shortlist, with the 24-year-old Hungary international valued between £10m and £15m, a similar figure which the Hammers are set to receive from the sale of Diop to Fulham.
With Fenerbahce having already allowed key defender Kim Min-jae to join Napoli this summer, it remains to be seen whether they would be willing to part ways with Szalai, who has previously been linked with both Chelsea and Atletico Madrid.
Since his arrival at Fenerbahce in January last year, Szalai has established himself as one of the top central defenders in the Turkish Super Lig, making a total of 67 appearances across all tournaments.
Finally, West Ham are also understood to have identified free agent Dan-Axel Zagadou as a potential target, following his release from Borussia Dortmund in June.
The 23-year-old has been hampered with injuries over the last two years and was reduced to only 15 Bundesliga appearances last season.
However, a number of European clubs are believed to be keeping tabs on the 6ft 5in defender, with The Sun claiming that the Hammers are set to face competition from the likes of Inter Milan and Roma for his signature.