The 29-year-old initially linked up with the Rams at the end of September before making his debut in a memorable 4-4 draw with Ipswich Town at Pride Park, in which he scored Derby’s opening goal.
Whitbread started the next two matches for Steve McClaren’s men – wins over Leeds and Watford, but a calf injury ruled him out until he returned to the squad for the Boxing Day draw away to Huddersfield Town.
The Texas-born defender has started four times for City this term, most recently in the 5-2 Capital One Cup victory over Carlisle United in August, and joined the Rams in search of further first team action.
Whitbread has made 21 appearances for Nigel Pearson’s side since signing from Norwich City in the summer of 2012 and scored one goal, which came against Derby in City’s 4-1 King Power Stadium win over their East Midlands rivals in December 2012.
Derby County currently occupy second spot in the league table, four points behind the Foxes ahead of a tantalising 10 January meeting between the two sides at King Power Stadium (KO 7.45pm) – a game for which Zak will be unavailable for selection.