Hull City assistant manager Dean Holden says winger Yu Hirakawa will see a specialist to determine the extent of an ankle injury.
Hirakawa came off in Saturday's defeat at home by QPR and faces a spell on the sidelines.
Holden said the extent of the injury will be determined on Tuesday but admits it is a blow to be without him.
"He is going to see the specialist," he told BBC Radio Humberside. "We'll have more information after that meeting.
"It was innocuous. He came off in a lot of pain with swelling in the ankle after he flipped it over.
"It is in the hands of the experts, we'll await the outcome. It is another unfortunate one for us.
"He has really impressed with his quality and energy. The fans have really taken to him, as have the boys. It'll be a big miss for us."
Holden also confirmed midfielder Matt Crooks is expected to be in the squad against Derby after stepping up his recovery.
"We expect Crooksy to be back in the squad. He trained this morning and looks OK. He is a good player and brings something different to the team.
"He's been out for a period of time and not trained a great deal. We'll have to see how many minutes he'll play."