The 25-year-old midfielder texted boss Brian McDermott on Monday saying his 'head wasn't right' then refused to travel to the north east for the 3-0 defeat at Stadium of Light the following evening.
His bizarre actions sparked a furious backlash from Royals fans, with some stating Guthrie should never play for the club again.
Now, in an exclusive Reading Chronicle interview, the former Newcastle United ace is begging for forgiveness ahead of showdown talks with McDermott today (Thursday).
However, Guthrie refuses to reveal the reasons why he chose to turn his back on the squad before Tuesday's basement battle.
He said: "I apologise to the fans. They will have their own opinions on the matter from what they have heard from other people. I admit, it doesn't look good for me from the outside but I won't go into details for reasons of my own.
"It's an internal matter that will be dealt with between myself and the manager. Personally I don't think these things should be spoken about outside of the club, which it has been.
"That is disappointing, but for me the important thing is to concentrate on improving our results.
"I don't want to go into the details any more than that but it's something the manager and I have to resolve."
McDermott is understood to have been furious and bewildered by Guthrie's decision not to fly to Sunderland with his Reading team-mates on Monday.
The fit-again midfield schemer was guaranteed a place in the squad at Stadium of Light and may have even started his first Premier League game since the 1-0 defeat at Liverpool on October 20.
It is not the first time Guthrie has been on the wrong side of McDermott and he was disciplined by the club for criticising his manager's tactics on Twitter earlier in the season.
Whether his show of remorse is enough to earn him a place in the team for the visit of Arsenal on Monday remains to be seen.
Guthrie stressed: "I can only apologise to the fans again. What happened doesn't put me in a great light. I don't think what I did was right, but I don't want to go into details. It's an internal matter between me and the manager and it should remain that way.
"The most important thing right now is the team. It is far more important than my little issue.
"All I will say is I am 100% fit and ready to play. It would be great to be involved in the Arsenal game, but we will have to wait and see."
Guthrie will almost certainly be fined and disciplined by McDermott, who can ill afford to cut adrift one of his most high profile players as Reading look to end a run of five straight league defeats.
"I don't know if he'll be in the frame for Arsenal but he was definitely in the frame for Sunderland on Monday," said a philosophical McDermott.
"To be honest, I don't know what else to say. He texted me to apologise for what he did but everyone knows you cannot do things like he did.
"He simply said his head wasn't right to travel but from my point of view that is not helpful at all.
"I think Danny realises that now. I believe it was a spur of the moment thing and I'm sure he regrets it.
"I will see him at training. It might be an awkward situation for him, but it's certainly not awkward for me."
And he added: "I need my players to do what players are paid to do, it's quite simple. They are paid to train the best they can and to play to their best ability when asked to play. And when they are not in the team they are paid to support those who are.
"Danny has let himself down badly."
++ BRIAN McDermott discusses Danny Guthrie's future at Reading - see website later ++