Rotherham manager Ronnie Moore is concerned by Ryan Cresswell's recurring back injury.
Cresswell had to pull out of Wednesday night's 3-0 FA Cup first-round replay defeat at York after suffering a back spasm in the warm-up.
It was the second time in a week that the 23-year-old pulled up with the problem after he was also forced off at half-time of last Saturday's win over Oxford.
"It's a big worry. He trained on Tuesday and it stiffened up in the warm-up and he wasn't able to continue," Moore said.
"We don't know what it is. It's a muscle spasm but he can't move his back. Considering we only have two fit centre-halves it's a worry."