Guthrie has apologised to supporters for withdrawing from the squad for the Barclays Premier League relegation clash at Stadium of Light, which the Royals lost 3-0, after claiming 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 and McDermott admitted his future at the Madejski Stadium hangs in the balance.
"I need to speak to him. We need to have a conversation," McDermott said.
Guthrie has been fined two weeks' wages by Reading for an incident that he insists is out of character.
"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 - 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."