Barcelona defender Sergio Busquets said Paris-Saint German were both the better team and played more physically in the French side's 4-0 Champions League triumph on Tuesday.
Angel Di Maria scored twice on his birthday while Julian Draxler and Edinson Cavani also added goals as PSG cruised to victory at the Parc des Princes.
Heavy favourites heading into the match, Barcelona must now become the first team ever to overcome a 4-0 deficit in the second leg to reach the quarterfinals.
"The were better than us," Busquets said of PSG, according to UEFA.com. "They played better, they were better tactically and they outdid us physically."
According to FiveThirtyEight's Soccer Power Index, Barcelona's odds of reaching quarterfinals dropped from 83 percent to seven percent after the first leg, but Busquets defended Barcelona's mentality and said he and his teammates would not give up.
"It's not about the attitude, it was a football thing," Busquets said. "PSG were better tactically. It will be very difficult at Camp Nou but we will try. We always try."
Andres Iniesta also rejected a claim that Barcelona were not mentally prepared for the game.
"I've never liked the word attitude and it doesn't go with this team," he said. "It's a question of football, of being well positioned, of not being beaten... They beat us playing football.
"We had a dreadful day in a competition that doesn't let you off. There's nothing positive, especially not the result. They were better than us in all facets of the game. There's no explanation.
"We were capable of getting things clear at any moment. Sometimes these things happen. And they hurt. Bad days and bad nights happen, but after you have to pick yourself up."
Barcelona manager Luis Enrique made no excuses for his team's performance.
"It was a disastrous game. It's not difficult to explain," he said. "PSG were better than us from the start. They overcame our pressure and they were better with and without the ball.
"We were clearly inferior. The responsibility falls solely on me. These players have given us a lot of joy."
Pressure on the coach has been starting to build with Real Madrid on top of La Liga, though Luis Enrique refused to discuss his future.
"Let's not waste time with easy topics," he said. "This is a passionate sport and there are always complications. The match was my responsibility.
"If you have to point fingers and single someone out, that is me as coach, since these are the same players that win on other days.