The Northern Ireland international was substituted at half time during the Millers' penalty shootout victory over Port Vale in the Papa John's Trophy after sustaining a knock in the first 45 minutes.
Manager Paul Warne is not sure if the striker will overcome the injury in time for Friday's second-round FA Cup fixture, as the Millers seek progress to the third round. Championship and Premier League sides enter the competition in round three.
"He took a crack early on that he never really got over," said Warne.
"I did see him getting his bottom strapped up, which the lads all took great delight in watching. We will see how he is for Friday."
The Millers will find out their last 16 opponents in the Papa John's Trophy when the draw is made on Saturday.
Michael Smith had put Rotherham ahead against Port Vale who equalised through David Amoo with three minutes remaining.
United scored all five of their penalties in the shootout with Viktor Johansson making the vital save.
Warne's side have now set a club record with Tuesday's fixture their 17th in a row without defeat, completely surpassing the 16-game run the club went on under Steve Evans in 2014.
“Port Vale are a good side going well in their league and there was no way they were going to lay down and let us tickle their bellies," added Warne.
“Our first-half performance merited the lead in my opinion, but we didn’t take our chances.
“We got an amazing goal in the second half but in fairness to Port Vale they kept asking questions and we didn’t have the answer to all of them.
“The goal came from a little bit of luck, but he put it away well and to be fair their play deserved a goal. All in all, it went to penalties and we’re really pleased to go through.”