West Ham could face FA Cup expulsion if Fifa finds the club breached rules over the selection of striker Diafra Sakho.
Sakho, 25, withdrew from Senegal's Africa Cup of Nations squad with a back injury but scored 18 days later in West Ham's 1-0 FA Cup win at Bristol City.
Fifa rules state a player cannot appear for his club if he is meant to be on international duty.
World football's governing body has begun an inquiry but the Hammers "vigorously" deny any wrongdoing.
If West Ham are found guilty, Fifa could order the Football Association to overturn their victory over the League One side.
Fifa is investigating a "potential violation" of its regulations by both the club and striker Diafra Sakho over his selection - but has declined to give a timescale on proceedings.
The Hammers are set to visit Premier League rivals West Brom in the FA Cup fifth round on Saturday, 14 February.
The Football Association is refusing to comment but is believed to be aware of Fifa's investigation.