Ronan Murray was sent off after 15 minutes of a seven-goal thriller at London Road on Tuesday night for throwing a punch at Danny Swanson.
By then Enoch Showunmi, with a rasping fourth-minute finish from an Alan Sheehan delivery, and Murray, who swept in a Kieran Freeman cross in the seventh minute, had fired the in-form visitors into a two-goal lead.
But their hopes of a fourth successive win were eventually dashed as Posh - spearheaded by the first hat-trick of back-in-favour frontman Nicky Ajose's career - made the most of their man advantage.
Ferguson said: "We won the game and that's all that matters at the moment.
"The confidence and belief is coming back after a couple of victories.
"The players showed tremendous character to come back from being behind. In fact it was like the good old days of seeing us turn a game around shooting towards the home end.
"I'm really pleased for Nicky because he has been very unlucky during his time here.
"I made my mind up to give him a run of games when I brought him back in from his loan spell at Swindon and he is getting the goals."