The midfielder apologised to McDermott and his team-mates after refusing to travel to Sunderland last month and was in the starting XI at Newcastle last Saturday as Adam Le Fondre's double earned a 2-1 win to ease the Royals' predicament.
"I'm really enjoying it and whatever has gone on in the past is clearly out of the window," Guthrie said on his club's official website, www.readingfc.co.uk.
"I felt like I owed everyone not just a performance, but a bit of form to get us out of the hole we were in. I feel like I'm slowly doing that."
Guthrie joined Reading from Newcastle last summer, but had made just five starts prior to the trip to Sunderland, which his side lost 3-0.
Now the 25-year-old has featured in the last seven games in all competitions as Reading have recorded three straight wins, against Crawley in the FA Cup, West Brom and the Newcastle victory, their first away win of the season.
Guthrie added: "I'm getting stronger in each game, and I got stronger as the game went on (at Newcastle).
"To put back-to-back wins together in this league is tough - especially in the position we're in. It's massive."