Warnock has moved to League One outfit Wigan on loan until the end of the season.
Warnock, who has played for Liverpool, Blackburn Rovers and Aston Villa among other clubs, joined Derby from Championship rivals Leeds United in January 2015, but the left-back has struggled to play regularly for the Rams.
The 34-year-old made seven appearances in the league for the Rams last season, creating four chances in the process.
The former England international defender had a pass accuracy of 76%, and won 22 headers, two dribbles and six tackles.
In 19 Championship matches for Derby this campaign, Warnock created 11 chances, had a pass accuracy of 82%, won 35 headers, seven dribbles and 38 tackles.