The 25-year-old was punished by Royals boss McDermott after the midfielder took to Twitter to voice his displeasure at being dropped for September's 2-2 draw with his former club Newcastle.
The pair were then involved in a training ground bust-up and Guthrie refused to travel to Sunderland for their 3-0 defeat last month.
However, Guthrie apologised in front of the whole squad and has since been handed a lifeline at the club.
"To do what he did took a big man," said McDermott.
"I was impressed with what he did, we then had a meeting and I said that to him in front of everyone.
"He's right back amongst the squad, staff, everybody.
"He instigated it all and it was very impressive. We have all moved on from what has gone on."
Guthrie has the chance now to show what he is capable of against his former employers when Reading travel to Newcastle today - knowing a win could drag McDermott's men out of the relegation zone.
"I'm really pleased with him," added the Royals chief.
"It's not easy to settle - he came from a big club to Reading.
"He's settled into the environment now and I know him better and he knows me better.
"He has had great support from his family and he is an important part of what we are trying to do here."