Posh's impressive start to the season continued as they triumphed 1-0 at the New York Stadium thanks to a second-half penalty from Grant McCann, with Rotherham blowing a string of chances as they suffered their first league defeat of the season.
Ferguson said: "They were the better team, there's not a doubt about that. In terms of chances they had more, there's no question.
"They were better on the day but we won the game and that's the most important thing. What I said in the dressing room made them fully aware what I think.
"The players are fully aware themselves we have to be better but the plus points are the fact we dug in and got the three points and that's the overriding factor.
"Players know I'm not going to accept those sort of performances but over the course of the season you're not going to play great football every week."
Rotherham's Iceland international defender Kari Arnason handled in the area and McCann scored the winner from the penalty spot.
It was an impressive response by the visitors after a half-time rocket from Ferguson following their sloppy first-half display.
Rotherham had the lion's share of the chances in the opening 45 minutes but failed to make the most of them and, after the goal, they produced a spirited rally but fell short of an equaliser.