Winger David Martin has joined Derby from Millwall until the end of the season on an emergency loan deal.
The 24-year-old, who played against the Rams in both FA Cup third-round ties last month, was due to join the Championship club before the close of the January transfer window but the move fell through.
Derby manager Nigel Clough told the club's official website: "David is a player we have monitored for the last six months or so, and we have been very impressed with him.
"He is naturally left-sided and will bring some pace and balance to the side.
"He also has a real hunger and desire to better himself and play at a higher level and we believe he fits the bill in terms of the type of player we are looking to bring to Derby County."
Martin is now set to make his debut against Championship leaders Newcastle at Pride Park on Tuesday evening.