Reid started in defence for the Hammers in a memorable FA Cup victory against Liverpool on Tuesday evening, but he picked up a hamstring injury in the contest.
The New Zealand international is now set to be sidelined for the next four weeks, in what is a blow to Slaven Bilic’s side.
Reid is one of West Ham’s most important defenders, and his absence significantly reduces Bilic’s options in defence.
Reid actually managed to play for just over an hour against Liverpool, but was forced to come off after picking up the hamstring problem.
He was replaced by James Collins, who looks set to fill in for him during his time on the treatment table, and could only watch from the sidelines as his teammates battled to a wonderful win.
West Ham had to wait until the 120th minute to get their winner, but when Angelo Ogbonna rose highest from a set-piece, he put the ball into the back of the net to send his side into the fifth round of the FA Cup, where they will meet Blackburn Rovers.