Hammers manager Sam Allardyce confirmed the details of the injury in his Friday press conference.
Carroll left Southampton on crutches on Wednesday night following West Ham’s 0-0 draw, in which the striker came on as a second-half substitute.
The injury has been scanned and the diagnosis is that Carroll could be out for up to six weeks.
“A full diagnosis on Andy is yet to come and we should have that today but it looks like a few weeks,” said Allardyce.
“The medial ligament on the inside of his knee has a little tear from a fairly innocuous collision with the defender.
“If we’re lucky it could be four to six weeks. It’s a big blow for him and us but one we’ve got to cope with.
“It is traumatic when someone is going through a period when in a short time??they overcome one injury and another occurs.”