Derby boss Nigel Clough hopes Jamie Ward will be back in first team contention before the end of the month.
Ward has missed eight matches since he limped off during the 1-0 victory over rivals Nottingham Forest at the City Ground on September 30.
The Northern Ireland international will miss the trip to face npower Championship leaders Crystal Palace on Saturday and might not be fit for the home game against Birmingham a week later.
Clough said: "Jamie is coming along slowly, we hope it won't be too long. He won't be back until he is 100% because he got involved [in training] a few weeks ago and felt it again.
"We hope he can get a game for the Under-21s against Barnsley on Monday but we are going to take our time with him and it might be more towards the Birmingham game, or maybe just after."