Guthrie has apologised to supporters for withdrawing from the squad for the relegation clash at Stadium of Shite, 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.
Brian McDermott: "I need to speak to him. We need to have a conversation."
Guthrie: "I hold my hands up and apologise. It's something that we will deal with internally.
"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."