Lampard was clearly unhappy with what he witnessed in the opening 45 minutes and admitted he could have substituted seven or eight of his players at the interval.
Barkley struck the game's only goal in the 63rd minute, ghosting on to Willian's cross to steer a shot home.
Chelsea record scorer Lampard's trust in youth has been an impressive part of his first season since returning as manager to Stamford Bridge, but he is not afraid to hand out some tough love when required, as it was on Sunday at the King Power Stadium.
"We haven't played that badly since I've been back, we were fortunate at halftime that it was 0-0," he said.
"The first rules of football are urgency and sprinting back and if you don't do that you can't compete. Our game needs to be brighter than that. In the second half there were glimpses of it, but we can't turn up like that again. We got lucky today."
Lampard said his young players would "take it on the chin."
"It is a learning experience. I could have taken more off, or other players off. I am not pulling those out on their own. They will be top players for this club and have top careers," he added.
"But if I have to do something in games I will do it. It was one of those games where I had to do something."
"Christian felt a tightness in his calf in the second drinks break, hopefully nothing that keeps him out," Lampard said.
Chelsea have won all three of their games since the restart of the season, twice in the Premier League to cement themselves in the top four, and can now look ahead to a Cup semifinal against Manchester United.