Both sides went into the match in search of a morale-boosting victory after failing to notch up wins in the Premier League so far this season.
QPR twice took the lead through Junior Hoilett and then Djibril Cisse, but saw their efforts chalked off by Kaspars Gorkss and an outstanding Nicky Shorey free-kick.
Former Fulham striker Pavel Pogrebnyak turned the tie on its head with a late backheel that saw the Royals progress to the last 16, despite him missing a stoppage-time penalty.
"I was proud of my team and the way they came back," said Reading boss McDermott.
"I talked about identity and the last two games in the Barclays Premier League against Spurs and West Bromwich Albion I just feel we have not performed to a level.
"Our identity as a team is what you saw tonight, what we're about, and I know if we perform at that level and that tempo with the way we pass the ball with pace then we are going to be more than capable in this league. No problem.
"I felt we needed to strip the team back to what we're about, really.
"In the last couple of games our tempo has been slow and we haven't getting crosses into the box or creating too many chances.
"We really stripped the side back. We had good conversations in the dressing room and we wanted to get a result and we did that tonight [Wednesday]."
Reading's reward is a home tie with Arsenal and, while pleased with the draw, McDermott's main focus is getting a first league win of the season this weekend.
"It is a good draw, we're really happy with it," he said. "I am sure we'll have a few people at the Madejski Stadium for that.
"We look forward to that but before that we have a big game against Newcastle that we're really looking forward to and tonight's result will be important going into that."
QPR also return to Premier League action, looking for a first victory against West Ham, after a night manager Mark Hughes when was left wondering what could have been.
"At 2-1 going into the last period of the game we certainly felt we had done enough," the Welshman said.
"We had to battle hard for that and credit to Reading because it was a different test today [Wednesday].
"We had to stand up to a physical challenge and they did a pressing game, which I thought for the most part we dealt reasonably well with certainly in the first half.
"We got the ball down and created a number of good opportunities that we didn't really capitalise on.
"Obviously at 2-1 I felt we were in the ascendancy and had done enough, but it was an outstanding free-kick from Shorey, which he always has the capability of and at 2-2 it was anybody's game.
"We are disappointed with the manner of the third goal because there were too many consecutive errors that led to the lad having space and time to be able to backheel it in our six-yard box.
"To lose it in that manner is disappointing but it is understandable to a certain extent given the changes and the injuries we have at the back.
"We haven't had that continuity at the back, which you need to bang results out.
"It is difficult at the moment because we're playing guys at different levels of fitness and we're trying to get everyone up to speed.
"That will come but we're just not there at the moment."
It had looked like a bad night for QPR had been compounded late on when Alejandro Faurlin limped off.
The Argentinian midfielder has only just recovered from an anterior cruciate ligament injury but Hughes believes he has avoided another serious blow.
"I have not had an update, in fairness," Hughes said. "I think he landed on his hip.
"As a consequence, he was struggling to walk because he might have banged a nerve or something but I am pretty sure it was not his knee, thankfully."