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 a disappointing night for us."