Britt Assombalonga's 97th-minute backheel earned the visitors a share of the spoils at Priestfield.
In-form Assombalonga had earlier slotted home his 14th goal of the season to put the visitors in front but they were pegged back after half-time by Joe Martin's free-kick.
And when Cody McDonald put the hosts in front it looked like Posh were going home with nothing, until Assombalonga bagged his second in dramatic style.
But Ferguson insisted his striker's last-gasp efforts should not mask a performance that needed to be far more clinical.
"It's a bad result for us, no question. It's two points dropped. Performance-wise, we were very good at times and some of our football was excellent," he said.
"But I think this month is really pivotal. We can't afford to do that this month.
"We need to be ruthless, that's the word I used to the players. We need to kill teams off. We weren't ruthless enough when it was 1-0 and that's cost us.
"I knew second half we'd come under pressure, I said that at half-time to the players, and from looking really comfortable we started looked really uncomfortable.
"I think we could have done better with the (Joe Martin) free-kick, I'll have to see it again, and the second goal we could have done better with.
"We kept on pushing and pushing and we deserved that bit of luck (with the late leveller). It was a striker's goal from Britt. He's only 20, he played lots of games at the start of the season, he had a break and he's come back stronger."