Derby forward Tomasz Cywka has undergone surgery to repair a fractured cheekbone.
The 22-year-old suffered the injury in an accidental collision with Cardiff defender Kevin McNaughton during the first half of the 2-1 home defeat on Saturday but battled on to play 88 minutes before he was replaced by Chris Porter.
Cywka went to hospital after the game and scans revealed the extent of the damage. He had an operation on Sunday afternoon.
Derby say it is too early to put a time-scale on how long Cywka will be out but he is likely to miss the trip to Midlands rivals Coventry at the weekend.