Nottingham Forest and Derby County are investigating after Richard Keogh was left bloodied after the visitors' Carabao Cup thumping at the City Ground.
Skipper Keogh was an unused sub in County's 3-0 loss at their arch-rivals on Tuesday night.
He later headed angrily for the dressing rooms after a warm-down turned ugly and security had to step in to split up a row with Forest groundstaff.
The flashpoint happened while Derby's subs were warming down on the pitch and members of the Forest ground staff were tending to the playing surface.
Republic of Ireland stopper Keogh, 33, was left fuming with blood running down the front of his neck.
Both Derby boss Phillip Cocu and Forest counterpart Sabri Lamouchi were giving pitchside interview's to the media when the incident occurred.
The clubs have declined to comment but are now known to in dialogue as they look to establish exactly what happened.
The incident is sure to ensure a red-hot atmosphere when Derby return to Forest in the Championship on November 9.
Forest were comfortably beaten on the night, after Cocu made 10 changes to the side that drew 1-1 with West Brom at the weekend.
Duane Holmes said: "We're all hurting - it was very quiet in the dressing room after the game.
"We're all upset with ourselves, because we know we've got to perform better than that, to be quite honest.
"The fans that have travelled are always brilliant and I feel like we owe them one."
