Derby County captain Richard Keogh is likely to be out for four weeks after a scan showed he damaged knee ligaments in Saturday's win over Doncaster.
The 27-year-old centre-half was replaced after 25 minutes and is not expected to return until late January.
Manager Steve McClaren told the club website: "It's obviously a blow. He is a very good defender and he's been in good form."
On-loan Manchester United defender Michael Keane replaced Keogh.
And McClaren is happy with the defensive cover he has.
"Injuries are part and parcel of football," he said.
"Michael came on in place of Richard and did well against Doncaster and we have Zak Whitbread and Mark O'Brien back in training too, so we're lucky to have cover in central defence."