The Hammers are currently in the fifth round of the competition after defeating Bristol City 1-0, however it is Sakho’s selection in that match – a game where he scored the opening goal – that has seen FIFA investigate the club.
Rules state that a club cannot field a player if he is meant to be on international duty. Sakho had earlier withdrawn from Senegal’s African Cup of Nations squad with a back injury but just 18 days later was scoring against Bristol.
The East London club “vigorously deny” doing anything wrong according to BBC Sport, but if they are found guilty of the offence they could see world football’s governing body order the English FA to overturn their result over the League One club in the fourth round of the Cup.
West Ham have said the inquiry by FIFA is “clearly unfounded”, however Sam Allardyce admitted he was told not to select Sakho for his side’s 2-0 loss to Liverpool at the weekend.
Bristol City meanwhile could claim up to £250,000 in compensation if the rule breach is confirmed but have so far refused to comment on the issue.