The Royals' previously watertight defence conceded three goals as they eventually went down 3-2 at Highfield Road this afternoon, despite an attempted late fight-back.
Reading's defeat, coupled with Ipswich's fine 3-0 win over Preston, saw the Royals drop out of the top two.
To make matters worse, leaders Wigan could race six points clear of Reading if they beat Leeds United tomorrow.
Midfielder Steve Sidwell, who has attracted the attention of a string of Premiership clubs following an impressive run of performances recently, came in for particular criticism from the disgruntled Coppell following today's game.
The 22-year-old midfielder felled tricky forward Stern John inside the box just moments after Ghanaian striker Lloyd Owusu had fired Reading ahead to give the home side an easy route back into the match.
Commenting on Sidwell's clumsy tackle Coppell said: "He's wiser than that, it was just one of those things and I don't think it will happen again."
Although the Reading manager was pleased to see his team pushing for a point in the closing stages of this afternoon's encounter, he admitted that he expected far more from his team, who have been touted as potential promotion challengers.
"Towards the end we were huffing and puffing and we've never been very good at that. I wasn't pleased with what we were doing but at least we were having a go.
"It reminded me a bit of the Cup game against Watford, when you're trying to get a goal back and give them chances on the break. Maybe if we had kept a solid shape we could have created more and given less away."