West Brom utility player Steven Reid will be sidelined for at least six weeks after suffering a torn cartilage.
Reid sustained the setback during Albion's 2-1 home reversal by Manchester United on Saturday and limped off in the second half.
The former Blackburn player had just started to establish himself at the forefront of Baggies coach Roberto Di Matteo's plans, but he will now undergo surgery and be out of action until at least the middle of February.
Di Matteo said: "It's really bad news for Steven. He is an important member of our squad and was playing well (on Saturday) until he was forced off.
"The big thing about him is that he offers cover for so many positions, but he's likely to be out for a minimum of six weeks. All we can do is wish him well with his surgery and a speedy recovery."
Reid made his reputation as a midfielder but has impressed in defence for West Brom.