Rotherham have been charged by the Football League with fielding an ineligible player, reports BBC Sport. Defender Farrend Rawson, on loan from Championship rivals Derby County, played the full 90 minutes as the Millers beat Brighton 1-0 earlier this month.
The Football League said a football disciplinary commission will hear the case at the "earliest possible opportunity" so the matter is resolved before the end of the season.
The Millers are currently seven points clear of the drop zone in the Championship but could have three points deducted.