Reading manager Veljko Paunovic accused his players of "immaturity" as the early Championship pacesetters slipped to a third successive defeat by losing 3-0 at home to Stoke.
Although Reading, who had won seven of their eight opening matches, dominated most of the game, defensive lapses proved costly against the resilient visitors.
Reading's attempted fightback failed to materialise and Stoke increased their winning margin in stoppage time through substitute Jacob Brown.
"The game started very well for us and we had [two] very good opportunities," Paunovic said.
"We have to learn that those moments are crucial in a game. You have to take advantage of it.
"This is a serious league in which mistakes – in both boxes – can kill you.
"There is a mix of immaturity in some moments and a mix of fatigue, accumulation of games and injuries that we are going through.
"I have to say, though, that there were still a lot of positives.
"The improvement and evolution in our game since we started the season until now, in terms of controlling the ball offensively, creating opportunities and dominating, is huge.
"But we slipped on the other side [defensively]. That is unacceptable and we will work towards improvement there in the next couple of weeks."
Reading have conceded 11 goals in their past four games.
"It's that mix of accumulation, fatigue and injuries," Paunovic added. "When you look at those three, that is what is affecting the team.
"We had a very good start today but maybe we got a little bit comfortable with the ball and then gave away two goals in the first half that had never happened to us in the past.
"That is the identity that we have to recover, of blocking those shots. Our doubters will now have negatives but we will fix this."
Improving Stoke picked up their fifth win in nine matches.
"I'm delighted with the three points," Stoke manager Michael O'Neill said. "We showed a different side to our game today.
"Reading were very good in the early part of the game and it took us a little while to settle into it.
"Angus [Gunn] made a couple of really good saves but then we created good opportunities and Tyrese took the first goal very well. He capitalised on their mistake.
"It was good pressure for the second goal as well. Steven takes it well. At that point, it looked as though we would get other opportunities.
"Of course, we're delighted with the three points, the three goals and also the clean sheet."
O'Neill paid tribute to Campbell's prowess.
"Tyrese has developed a lot and there's a lot more to go still," O'Neill said.
"He's got fantastic natural attributes in terms of his speed and his physicality. And he's a great finisher.
"So those are three boxes to tick. But he still needs to find a little bit more in certain areas. I'm constantly at him.
"We found a way to get him into the team in a wider area, like a wide forward. Not a winger but a wide attacking threat.
"I don't think that he's ready to play as a lone striker but, when he plays up there with Fletch [Steven Fletcher], he seems to enjoy it as well.
"I'm delighted with the progress that he's made."