The 25-year old midfielder refused to make the trip to the Stadium of Light as he had not featured for the Royals since October and now Sky Sports suggests he has been fined two weeks' wages for the incident.
"He made a decision that his head wasn't right to travel," McDermott said.
"Everyone has got to be going in the same direction, that's really, really important for me as a group, and we have had that for a period of time.
"You have to put yourself up to be selected in each and every game, and that means every player.
"Any player has to be part of what we are about. We can't deviate from what we have been about over the last three years.
"We are very much about the group - we have to be. We all have to pull in the same direction. It's probably not a conversation for this evening, but it's something that we need to address in the week."