West Brom have confirmed that Jonathan Greening is set for a lengthy spell on the sidelines.
The club captain suffered medial ligament damage to his left knee during Saturday's FA Cup clash with Burnley.
The 30-year-old lasted just 40 minutes of the 2-2 draw at The Hawthorns before being forced off.
He has now undergone medical tests which have revealed the severity of the problem.
The injury is expected to keep Greening out of action for at least a month, but could rule him out for up to six weeks.
West Brom boss Tony Mowbray will be disappointed at the news, with the club currently battling for their Premier League status at the wrong end of the table.
An Albion spokesman said: "Scan results confirm that Jonathan Greening strained the medial ligament in his left knee against Burnley.
"Greening is expected to be sidelined for between four and six weeks."
Mowbray added: "Jonathan has been our most influential footballer in my time here. Him being out for any amount of time is a big blow to us.
"It is his first injury of any note during his career."
However, Mowbray has said he will not bring in additional players on loan to the ones he is already targeting to fill the gaps left by Greening and the other posse of injured players.
He continued: "We won't be changing tack or going out and bringing anymore players in because of the injuries.
"I think it is the wrong thing to do, to bring a loan player in to fill a space for what might only be a week or two in the case of some of the injured players.
"If you bring loan players in now, you are making a commitment for the next five months really. I don't see it being the right thing for the player coming in, who might play a game or two and then people get fit and they are left out.
"We will go along with the squad we have at the moment, intensify our efforts regarding the players we are already trying to target in certain areas of the pitch, and see if we can make things happen this week."