A second straight defeat has turned the pressure back on Mourinho's Chelsea side, with big-spending PSG now favored to reach the last four in the European Cup for the first time since 1995 and Liverpool grabbing the impetus in the Premier League.
''When the games are tight and when you make the defensive mistakes, you are in trouble,'' Mourinho said. ''We couldn't transform these half chances into goals and on top of that we made defensive mistakes, the kind of individual defensive mistakes, and we paid the price.''
PSG substitute Javier Pastore capped the win with a brilliant goal in stoppage time. The Argentina playmaker gathered the ball on the right wing, cut inside and then jinked his way through the penalty area before planting a low shot inside goalkeeper Petr Cech's near post.
''It was a horrible time to concede,'' center half Gary Cahill told British broadcaster Sky Sports. ''It's a sloppy goal and unlike us. It was difficult to come here; 2-1 would have been okay to take to Stamford Bridge. It's devastating.''
Mourinho was more scathing.
''He (Cahill) says sloppy. I say ridiculous,'' later adding. ''It's not a goal, it's a joke.''
He also criticized his forwards, with neither Andre Schuerrle or his replacement, Fernando Torres, making any impression in the absence of the injured Samuel Eto'o.
''It's difficult for us to score goals,'' Mourinho said. ''I'm not happy with my strikers' performances and I have to try things (for the return leg).''
Chelsea did well to contain Zlatan Ibrahimovic, PSG's top scorer with more than 40 goals this season. But Jose Mourinho's team was undone by lapses from captain John Terry and midfielder David Luiz, who gave the ball away leading up to the opening goal.
It came after only four minutes when Ezequiel Lavezzi twisted to sweep the ball home after Terry's poor headed clearance. Eden Hazard equalized from the penalty spot midway through the first half, only for PSG to take the lead again in the 61st as Luiz shinned the ball into his own net.
Chelsea won the competition two years ago, although Mourinho was not in charge, getting past Napoli despite losing their quarterfinal first leg 3-1.
''I have nothing to lose now, I have to play thinking that it is possible,'' Mourinho said. ''To be fair, I don't think Paris think that it's done.''
PSG coach Laurent Blanc was full of praise for his players.
''We really believed we could do it tonight, and we pushed Chelsea around at times,'' he said. ''The third goal changes things. We can't say tonight how much it changes things. The match at Stamford Bridge will be difficult.''
There was some bad news on the night for PSG, however, with Ibrahimovic limping off midway through the second half with a hamstring injury.
''He will probably be out for a while,'' Blanc said. ''I hope he'll be back as soon as possible.''
In Wednesday's other quarterfinal first leg, Real Madrid beat last season's finalist Borussia Dortmund 3-0 in Spain.
PSG started aggressively and scored when Terry failed to deal properly with Blaise Matuidi's cross and his header fell to Lavezzi some 15 meters out. The Argentina winger swiveled brilliantly before planting the ball into the top left corner for his 10th goal of the season.
Chelsea struggled to deal with PSG's speed on the break, and midfielder Ramires was shown a yellow card after hacking down Lavezzi as he turned away from him near the halfway line. Ramires misses the return leg next week.
Lavezzi caused Cahill constant problems with his pace. Midway through the half, Lavezzi got away from him down the left but his shot hit the side of the net.
Chelsea equalized against the run of play in the 27th minute when Brazil center half Thiago Silva lunged in a fraction too late and clipped midfielder Oscar's leg. It was surprisingly rash judgment from Silva, the Brazil captain, who is considered one of the best defenders in the world.
With the jeers from the Parc des Princes crowd ringing in his ears, ice-cool Hazard sent goalkeeper Salvatore Sirigu guessing the wrong way.
Lavezzi had another effort saved by Cech, but the home side nearly fell behind in the 40th minute when Hazard's brilliantly controlled volley from wide left hit the right post.
Lavezzi outshone Ibrahimovic and his looping free kick caused panic in the Chelsea defense and Luiz clumsily knocked the ball over the line.
Worse was still to come for a ramshackle Chelsea defense that will need a vast improvement to stop PSG scoring an away goal next week that would surely put the tie beyond the Blues' reach.