Nottingham Forest defenders Kelvin Wilson and Danny Collins are both expected to miss three months of action.
Wilson, who returned to the City Ground in a summer switch from Celtic, was substituted in the 2-2 draw with Middlesbrough in September with a back problem.
He returned off the bench in the 1-1 draw with Bournemouth a fortnight ago, replacing Collins, who suffered ankle ligament damage and will now be unavailable for 12 weeks.
Wilson aggravated his injury in the match against the Cherries and has not featured since, forcing Forest boss Billy Davies to send him for a minor operation that will leave him sidelined for a similar amount of time to his fellow centre-back.
Jack Hobbs and Jamaal Lascelles played in the middle of Forest's defence in their 1-0 home defeat to Blackpool on Saturday.
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