Smith, who Fulham signed from Leeds for £500,000 on Deadline Day in August, took to Twitter to wish his former team-mate all the best at Derby.
And ex-Leeds target man Smith has congratulated Warnock on the new chapter in his career.
All the best to @StephenWarnock3 in an exciting new chapter at Derby! One of the best I've ever played with & an absolute gent to match— Matt Smith (@mattjpsmith) January 15, 2015
Smith has since moved from Fulham to League One side Bristol City on loan, where he is finding the net regularly.
The 25-year-old found game time hard to come by at Craven Cottage before making the temporary switch.
Leeds, meanwhile, will likely turn to youngster Charlie Taylor to fill the void left by Warnock, who will be hoping he can celebrate promotion with Derby come the end of the season.