Jamie Ward is unlikely to feature for Derby again before the international break after suffering a hamstring injury in the victory over Nottingham Forest.
Ward limped off after 66 minutes of the Rams' 1-0 win at the City Ground on Sunday afternoon.
The 26-year-old will undergo a scan to determine the extent of the damage.
Derby travel to Middlesbrough on Wednesday night and face Brighton at Pride Park on Saturday before a two-week break for the next round of international matches.
"It's a concern for us but we'll have Jamie assessed and see what the damage is, but it is obviously a blow," Rams coach Johnny Metgod told the club's official website.
Ward is County's top scorer this season with four goals.
He has also been in the Northern Ireland squad for recent matches.