Derby suffered defeat in the Championship for the first time this season, and McClaren believes that it was only one area which let the side down as Charlton continued their impressive start to the new campaign.
"You can't win a game if you concede three goals," the Derby boss told reporters. "We've had three clean sheets before this, but one of our strengths has let us down.
"The boys showed fantastic attitude, kept going, fighting, creating, but three goals to come back from is a difficult task.
"That's what we're struggling from. Charlton have found a rhythm easily, consistently. Margins are very fine."
Derby will look to return to form when they host Fulham on Saturday afternoon.