Posh fell behind at the Reebok Stadium to Craig Dawson's fifth-minute header, the defender's third goal in two matches, and failed to find an equaliser despite enjoying greater possession after the interval.
Ferguson's side remain four points from safety and the manager admitted he was perplexed by their lack of conviction.
"That's two games in a row now that our final ball has not been good enough," said the Scot. "If I'm going to be honest, I didn't see us scoring.
"I think we lacked that little bit of belief that we could get anything from the game, which is unusual. In the second half we got control of the game, but never really had many openings."
Ferguson is convinced Peterborough can still stay up, provided the squad embrace the task that lies ahead of them.
"We need six wins and we need them quick, but my players can win games," he added. "We can win games against any team in this league, we've shown that.
"This is what you're in the game for. This is the business end of the season.
"It's the bit I enjoy and it's the bit the players have got to enjoy.
"They have to enjoy the challenge of trying to keep the club up because the difference, and no disrespect to League One, between this league and the one below is absolutely gigantic."
- Sports & Recreation