Cahill laments late goal
Substitute Javier Pastore was able to wriggle free of some lax defending and fire past Petr Cech's near post with the last kick of the game and leave Chelsea staring at elimination ahead of next week's second leg.
"It was a horrible time to concede," Cahill told Sky Sports 1.
"It's a sloppy goal and unlike us. It's a big blow at 2-1.
"It was difficult to come here; 2-1 would have been okay to take to Stamford Bridge. It's devastating."
Cahill's boss Jose Mourinho was more scathing in his assessment of the defending.
"He (Cahill) says sloppy. I say ridiculous," he added.
"We played against a great team, full of good players especially attacking players.
"You would expect them to score fantastic goals. Not the goals they scored."
Chelsea had been worth the 1-1 half-time scoreline but were over-run after the break. David Luiz gave away a soft free-kick and then bundled Lavezzi's dangerous free-kick into his own goal, before Pastore's late blow.
"We lose the first goal in an easy position and we assist the striker," said Mourinho.
"Nobody in second goal was there for a crucial position. The second goal one of my players scores an own goal, it's unlucky, but the way the team positioned themselves defensively was not the right one.
"Finally the third goal was sloppy, Gary uses that word, but for me, ridiculous."