Two second half goals from Henri Camara and Darius Henderson gave the Blades a 2-0 success at Brammal Lane on Saturday afternoon.
"We're a bit fragile at the moment," Johnson said after the match.
"We weren't pulling up any trees but we were holding our own and then the first goal killed us.
"I thought the start was pretty even but towards the end they were busier and cleverer than us.
"We fell behind because of poor defending. It was a schoolboy error."