Derby boss Wayne Rooney admitted his frustration after seeing Peterborough score twice in added time to condemn County to a 2-1 Championship defeat.
Darren Ferguson's newly-promoted side staged a stunning late fightback at the Weston Homes Stadium as Siriki Dembele struck the winner 10 minutes into stoppage time.
Nineteen-year-old Jack Stretton's first goal for Derby, a fine finish from 15 yards following a Graeme Shinnie corner, put the visitors in front, but Harrison Burrows tucked away a Joe Ward cut-back to level in the 91st minute before fellow substitute Dembele completed the turnaround right at the death.
Rooney said: "I never felt in any danger at all throughout the 90 minutes, but unfortunately lapses in concentration and individual mistakes have cost us in injury time.
"It's frustrating to concede two goals in the way we did after being the better team overall and deserving the win, but that's football. You don't keep your concentration you get punished.
"We put a lot of work in and yet we go away with nothing to show for it.
"As a manager you can cope with players making mistakes. What you can't deal with is them not trying and every single one of our lads gave everything for us."
Substitute Stretton came on for Colin Kazim-Richards, who had to be taken off on a stretcher, and Rooney said the former Turkey international was likely to be out for some time.
"It's too early to say what the extent of Colin's injury is, but I don't think it is going to be a quick fix," Rooney said.
"We'll lose him for a while and that means we now have to rely on a young kid in Jack to play 90 minutes in every game.
"It's obviously not great for him or ideal for us, but we know the position we're in. As I keep saying, it's going to be a tough and long season."
Ferguson praised his players for producing a grandstand finish.
He said: "The players deserve a lot of credit to turn it round and a get result.
"We knew the injury time would play a big part in the game as it would get the team going who were behind.
"That was us and as soon as we got the equaliser I felt something good could happen.
"Harrison got himself into a great position to bring us level and it was a really good, composed finish from Siriki at such a key moment in the game.
"We have a great record of coming back in games and it's vital we are able to keep doing that and picking up points at home.
"I was getting frustrated as I thought we lacked a bit of belief for a lot of the game. We conceded a sloppy goal to fall behind and that left us with no choice other than to take risks and to go for it.
"The Championship is a relentless league and I stressed to the players it was important to get some points on the board from this game so I'm delighted with how we did it."
