Birmingham defender David Murphy will be sidelined for at least six weeks due to an exploratory knee operation.
The left-back's absence has prompted the Blues to extend former West Brom and Bolton defender Paul Robinson's short-term deal at the club for a further two months until January 27.
Murphy, 28, was forced out of last weekend's home defeat to Hull and will now undergo surgery to find out the full extent of a cartilage problem.
"We won't know exactly what the situation is until Murph has had the operation on Thursday evening," said manager Lee Clark told the club's official website.
"He's damaged his cartilage and it could be anything from six weeks to five months.
"Obviously we're hoping that it's not very serious because we all know he's a very important player to us. We will have to wait and see."
Clark has recently lost defenders Stephen Carr (knee) and Paul Caddis (shoulder) to long-term injuries, while Pablo Ibanez has just recovered from a broken foot and Curtis Davies is still struggling with an ankle problem.