The 33-year-old only joined the Rams in June on a two-year contract after leaving West Bromwich Albion, but has failed to hold down a regular place in the side.
After making just a single Championship appearance for the promotion contenders in 2016, he has rejoined his former club Fulham for the remaining three months of the campaign.
The Northern Irishman previously played 165 matches across six Premier League seasons with the Cottagers between 2007 and 2013 before falling out of favour at the club.
However, he had played just 16 times in all competitions for County in his first season at Pride Park, starting only eight Championship matches.
Derby are sixth in the Championship, seven points off an automatic promotion place, whilst Fulham are embroiled in a battle at the other end of the table, six points clear of relegation in 19th.