Peterborough manager Graham Westley says he made an error by bringing midfielder Jack Collison on for Chris Forrester in the 3-2 loss to Coventry.
Forrester was taken off after being sick at half-time with Posh 2-0 ahead.
"What I saw was a young team that in the second half lost its way," Westley told BBC Radio Cambridgeshire.
"If you look at the team I constructed by my substitution I probably took away a bit of athleticism at a time when some of the younger boys tired."
He added: "I've made a mistake in the way I changed the game - it was a forced change but nonetheless I made a mistake.
"And I, like them, am learning about the changes that will and won't work. But there's so much good in that dressing room that it's very difficult to get downhearted."
Posh have now won four and lost three of their eight games since Westley replaced Dave Robertson at the ABAX Stadium.
But Westley did reserve praise for his side's first-half performance, when Jermaine Anderson and Erhun Oztumer had put them ahead.
"I thought the boys gave so much to the game this afternoon," he said. "The first half we were brilliant with the football, we worked it so well from the back, and got ourselves probably a deserved advantage.
"After half-time, the change altered the way we played - Forrester gives you so much in that his mobility and rotation is excellent. He gets us on the ball and passing.
"We didn't pass the ball in the second half. We were obviously going back to front far too often as a consequence of the lack of movement - you can't play a midfield diamond and not have that mobility."