The Portuguese player was taken off during the first half of the Liberty Stadium clash and his absence means Hammers boss Sam Allardyce's options in attack are stretched even further.
The club has not put a timescale on his recovery but the 27-year-old suffered a similar injury last season that kept him out for three months.
Allardyce is already without big-money signing Andy Carroll, who has not played this season because of a foot injury.
But there was a boost for the Hammers boss in Sunday's 0-0 draw when Carlton Cole came off the bench for his first appearance since returning to the club, and the former England striker is happy to be back.
"It was really good to be out there again," Cole told West Ham TV. "I was disappointed we didn't win but we were away from home so you can't ask for much more than getting a clean sheet.
"The boys have been doing well and we've got to give credit to them for only conceding one goal away from home so far this season.
"It's a huge credit to all the playing staff and the management as well.
"I'm chuffed to get back and to be involved in it again. Any footballer will tell you that you have to work hard and the manager has noticed that I was working hard and has given me a new deal.
"Hopefully I can push on from here and add to the team and play a significant role."