The 25-year-old, a summer signing from Newcastle, did not travel to Wearside after telling McDermott his "head was not right", prompting a withering response from his manager.
McDermott said: "He made a decision that his head wasn't right to travel. 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."
The Royals boss continued: "You have to put yourself up to be selected in each and every game, and that means every player.
"His words were his head wasn't right. He would have been travelling."
Asked after the 3-0 defeat at the Stadium of Light how he would handle the situation, McDermott said: "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 (straight after the game), but it's something that we need to address in the week."