Darren Ferguson said his team's defending for Sheffield United's winning goal was worthy of an Under-12s side after Peterborough went down 2-1 at London Road.
Ben Davies' drilled strike with six minutes to go earned the visitors their first points of the season, following a first-half opener by the Blades' Jose Baxter.
Shaun Brisley equalised nine minutes later but Ferguson, who gave a second-half debut to Christian Burgess, was disappointed at how his side lost the match.
"If we had gone ahead we would have gone on to win," said Posh's boss, who revealed the club will not increase their bid for Hartlepool's Luke James.
"We spoke about how we had to defend when they put balls in the box. It was a really poor goal to lose but we will learn
"If you are not going to win at least do not lose. We did that too many times last season and it's a habit we have to get out of.
"You cannot lose the game as we have. It's Under- 12s stuff.
"It's frustrating. I thought first half we were outstanding and they could not live with our football.
"We should have been out of sight, but we switched off for one moment. I said at half time we need to go again. Having four centre-halves injured is testing, but I'm not one to make excuses.
"We trailed Christian for a while and he will be a good bit of business for the club."
Posh started well but Baxter gave the visitors the lead in the 25th minute when he slotted across Ben Alnwick.
Brisley headed in Jon Taylor's cross nine minutes later, but Baxter's drilled effort from 20 yards won it after the hosts had failed to clear a long throw.
"I thought we started a bit nervy and there was a lack of confidence," said Blades' boss Nigel Clough.
"But I was disappointed we did not go into ahead at half time. It was a soft goal to concede and we think Mark Howard was impeded.
"If you watch us in training you would see that sort of thing [Baxter's goal]. That's what we work on. It was a very good team goal.
"Ben Davies has been playing out of position at full-back, but on the right-wing I think he was our best player. If you want that ball to land to anyone's feet it's him.
"Our work-rate was up from Saturday with the challenges going in.
"We did not play nearly as well as the first 30 minutes against Coventry but we did the other bits well.
"Peterborough will be top six at the end of the season and we feel now how Bristol City felt when winning at our place."