Jose Mourinho's side are chasing a two-goal deficit in the Champions League quarter-final second leg at Stamford Bridge after conceding in stoppage time at Parc des Princes
PSG took the lead at Parc des Princes through Ezequiel Lavezzi, only for the visitors to pull level through an Eden Hazard penalty.
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"[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 it's a disappointing night for us."