He returned to action in the friendly against Ilkeston Town at the beginning of October but has struggled with niggling injury problems since.
Zadkovich was signed by former manager Paul Jewell and his contract is set to expire at the end of the season.
"Ruben is still injured and he has been offered a termination agreement for the last six months of his contract. We are waiting for his agent's response on that one," Rams boss Nigel Clough told the Derby Telegraph.
Zadkovich has made just seven appearances since joining Derby in April 2008.