Guthrie refused to participate in Tuesday's 3-0 defeat at the Stadium of Light - a result that leaves the club second from bottom in the Barclays Premier League - because his "head just wasn't right".
The 25-year-old, who has recently recovered from a thigh injury, has played just six matches since his free transfer from Newcastle in the summer but McDermott is ready to move on from the incident following the player's show of contrition. "Importantly, we can all put this to bed now," McDermott said in a press release.
"Every person in life makes mistakes and Danny apologised to all of his team-mates and the staff in a group meeting. He asked me if he could speak to the entire group before training, it took a lot of courage for him to do that and he deserves respect for his reaction.
"Now myself, the players and staff will all move on. We have 22 games ahead of us and together as one unit we can achieve our goals for Reading Football Club and for you the supporters."
Guthrie, who has been fined two weeks' wages for an incident that he insists is out of character, regrets the incident.
"I hold my hands up and apologise. It's something that we will deal with internally," Guthrie told the Reading Post. "There's no problem with anyone. I have my reasons for what happened, it shouldn't have happened, but it did. My head just wasn't right. I love being at this club and I just want to play football. I've never had problems anywhere I have been and I just want to get back to playing football. This is a very rare situation for me."
The specifics of Guthrie's refusal to travel to the Stadium of Light have yet to be disclosed, but McDermott insists the needs of the squad will always supersede those of the individual.
"The definition of a good team player is to leave your ego to one side for the good of the team. That is why we have been so successful," McDermott said. "Everyone wants to play and I understand that, but when you are a professional footballer you are paid to play when you are asked and when you are not playing you support the team.
"That will never change as long as I am here. We've had incidents over the years, but in this league everything gets magnified. There is a lot of talk about this rather than the football side."