Derby defender Russell Anderson has suffered a setback in his return to fitness.
The centre-half was due to resume full training on Monday after a two-month lay-off with a hamstring problem.
However, he sustained a calf strain during his rehabilitation work last week.
"It is a blow for Russell, I feel for him," Derby manager Nigel Clough told the club`s official website.
"He has had such a frustrating time this season and now he will be out for a few more weeks. It could be up to four weeks before he is in a position to join full training again."
Anderson suffered a groin injury on the opening day of the season before a hamstring injury sidelined him again soon after his return in October.
The 32-year-old has been restricted to just four appearances this season.
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