West Ham United could face expulsion from the FA Cup if FIFA finds that they have breached rules over the selection of striker Diafra Sakho, who netted the winner in their fourth round win at Bristol City.
Sakho withdrew from Senegal's squad for the Africa Cup of Nations with a back injury, only to return just 18 days later to guide the Hammers into the fifth round.
FIFA rules declare that a player is not allowed to appear for his club if he is meant to be on international duty.
An investigation has been launched by the governing body, while West Ham "vigorously" deny any wrongdoing.
If found guilty, West Ham could see the result overturned in favour of the League One outfit after what was a tightly contested game.
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Senegal were said to be upset that Sakho had appeared for West Ham just 48 hours before they were dumped out of the Africa Cup of Nations at the group stage.
Due to the investigation, key striker Sakho missed the Hammers 2-0 loss to Liverpool at the weekend and unless anything is resolved this week, is set to sit out Sunday's home game against Manchester United.
FIFA have not set any time scale on their investigation.