Rhys Williams is set for a long spell on the sidelines following confirmation that he has damaged ankle ligaments.
The Australian defender collapsed in the second half of Middlesbrough's Capital One Cup win at Gillingham on Tuesday.
A scan has revealed serious damage, and Boro boss Tony Mowbray accepts that he will have to do without his captain for a lengthy period.
"He's had scans this morning and he's damaged his ankle ligaments, which was what we suspected," said Mowbray.
"He will now see a specialist to decide if he needs an operation or not but it's still going to be a considerable time out.
"It's a blow both to Rhys and to the club. We were going to depend on his quality and athleticism this year. We were hoping he was going to develop a strong relationship with Jonathan Woodgate.
"But he's out injured, and we have to take that on the chin. He has to go through rehab and come back stronger. It's going to be a month or two rather than weeks."
Woodgate may feature in Boro's match at Millwall on Saturday after missing their last two matches.
Meanwhile, Mowbray is assessing former Reading and Burnley centre-half Andre Amougou, having given the Cameroon international a trial at The Riverside.