Former Glover loanee Adam Rooney opened the scoring before substitute James Collins added a second in the dying stages.
The result saw Johnson's side drop out of the play-off places and the manager wants to see his side bounce back and secure a top-six spot.
He said: "I felt the first half was pretty tight and we needed to stick one of our chances away, which would have settled us down a bit.
"We had some inexperienced players out there who I think showed that inexperience on the big stage. We didn't have all of them, especially in the second half, doing enough.
"On the big occasion they weren't quite up for it. It was really disappointing in the second half, but we must not let our season fade away. I'm not going to let that happen.
"I will tell them home truths as I always do, and a few of them didn't show that they knew the importance of the game in their body language.
"We need to make sure we fight back in the next couple of games and get back in those play-offs."