Jose Mourinho's side has a two-goal deficit in the Champions League quarterfinal after conceding in stoppage time at the Parc des Princes
PSG took the lead at the Parc Des Princes through Ezequiel Lavezzi, but Chelsea pulled level through an Eden Hazard penalty.
But a David Luiz own goal put the hosts back in front, before Pastore beat Petr Cech at his near post at the death to give his side a huge boost ahead of the Champions League quarterfinal second leg at Stamford Bridge.
"[It was a] horrible time to concede then, a sloppy goal, unlike us and it was a big blow at 2-1," Cahill told Sky Sports.
"Obviously it was difficult to come here. 2-1 would have been okay going back to Stamford Bridge I suppose, but the blow at the end is devastating."
Cahill remains optimistic that Chelsea can still win the tie at Stamford Bridge, despite the two-goal deficit.
He added: "It will be tough but we're at home at Stamford Bridge and we've been playing well there.
"We need to, we've got a big task ahead of us and we need to approach it in the right manner and be on the front foot, but obviously a disappointing night for us."