Sheffield United defender Chris Armstrong faces a spell on the sidelines after undergoing surgery on his knee injury.
Chris Armstrong faces a spell on the sidelines
The 24-year-old missed Saturday's 1-0 win at Newcastle United and now requires an operation.
Blades' boss Neil Warnock said: "There remains the possibility that if it is a just a piece of floating bone Chris could be out for only three weeks."
But he admitted: "If it turns out to be a cartilage problem, it will take six."
Armstrong had re-established himself at the back for United, having overcome a career-threatening problem in the same knee.
Warnock added: "It is unfortunate for Chris and us - as it is for Michael
Tonge, who is out of action with damaged ankle ligaments at the same time.
"But I am confident that both will be back as quickly as possible challenging
for places again."