Royals boss Brian McDermott had confirmed that Guthrie had made the decision not to travel to Wearside with the rest of the squad for a game they lost 3-0.
Sky Sports revealed that Guthrie had been fined two-weeks' wages by the club - and now the former Newcastle United man has issued an apology.
"I hold my hands up and apologise. It's something that we will deal with internally," he 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."
Guthrie now insists he wants to put the matter behind him and battle for his place.
"I love being at this club - I just want to play football," said the 25-year-old.
"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."