Sakho limped off during Sunday's 1-1 Barclays Premier League draw with West Brom with a thigh problem and reports on Tuesday suggested the Senegal forward was out for three months.
But the exact prognosis is still to be determined after Sakho was injured stretching to shoot into the side-netting during the second half at Upton Park.
West Ham boss Slaven Bilic said afterwards: "It doesn't look good
It's a big blow for us, he's a key player."
His is the third injury to hit West Ham's attacking resources after playmaker Dimitri Payet and winger Enner Valencia were hurt against Everton earlier this month.
"You can see injuries in every club and to be fair the number who are missing is not a lot, but the ones missing are key players who play in the same position," added Bilic.
"Valencia, Payet, and now Sakho
It is very hard to replace them."
The Hammers play Manchester United at Old Trafford on Saturday.
Source : PA
Source: PA