Crawley wrecked high-flying Posh's best start to a season for more than half-a-century with a 2-0 triumph at London Road where a brace from veteran striker Gary Alexander did the damage.
Alexander broke the deadlock with a terrific volley from a tight angle in the 22nd minute after being picked out by a Billy Clarke cross.
And the 34 year-old was on hand to double the advantage when punishing some pitiful defending in the 64th minute.
Posh posed little threat as their six-match winning streak came to an abrupt end, but Ferguson was also quick to praise the performance of conquerors Crawley.
Ferguson said: "We didn't play with anywhere near the energy or tempo that I expect.
"We huffed and puffed, but never looked like getting back into the game after going behind.
"It's been a really good start, but we still have to get better at getting something out of games when things aren't going our way.
"I need to see a reaction from the first minute of training on Monday morning.
"The good thing for me is I have a strong squad and plenty of players waiting in the wings for an opportunity.
"All credit to Crawley though as they are a good team and I think they will be up there this season."
Striker Britt Assombalonga saw two strong shouts for a penalty ignored by referee Darren Bond in the closing stages.
Bond did dismiss Crawley man Mike Jones late on for collecting a second caution, but that did not stop the visitors coming within a whisker of a third goal when substitute James Hurst rifled inches wide.