Emanuele Giaccherini helped to launch the post-Paolo Di Canio era with a 32nd-minute strike and a second-half header from substitute Valentin Roberge handed interim boss Kevin Ball a win at the first attempt and just a second of the season for the Black Cats in all competitions.
Ferguson was left to reflect on a poor performance from Posh in the first half, which allowed Sunderland to establish a momentum, and insisted events at the Stadium of Light over the last few days had not come into his thinking.
He said: "I can't control what Sunderland do. They have made a decision - first and foremost, I have got to make sure my team performs to the level we can, and we didn't first half.
"The players know that, they are disappointed with themselves. That was all we concentrated on.
"We spoke about certain things Sunderland might do and their strengths and what we felt might be weaknesses, but I have got to make sure my teams plays to a level we know we can and in the first half, we didn't, it's as simple as that.
"We were in the game. We felt the longer it went at 1-0, perhaps we had more of a chance because we know it can get quite frustrating for the fans here. That's evident, especially at the moment.
"But to lose two goals from two crosses is poor, we shouldn't be losing goals like that."