PSG dominated possession and created the best chances at the Parc des Princes but conceded two goals because of individual errors before a contentious penalty from Marcus Rashford in the fourth minute of stoppage time. United won 3-1, eliminating PSG from the last 16 for the third straight year.
"For me it was an accident," Tuchel said ahead of Tuesday's French league game at Dijon. "I know there have been many accidents over the past years. But it's the first time with me. We made two individual mistakes, then the referee made a mistake with the VAR."
PSG, which has spent a huge amount of money over the past five years in its unsuccessful bid to win the Champions League, has found itself at the receiving end of sharp criticism since the loss to United. Trying to find an explanation for the result, some blamed a culture of losing big games, while PSG defender Presnel Kimpembe said he and his teammates were too complacent.
"We lost the grit, the rage we had in the first leg," he said. "We took this game lightly, and it's something that'll never happen to me again. It's enough, never again. Of course, we'll lose more games in the Champions League, but losing games like this is over. I don't want it to happen to us anymore."
Tuchel said he felt the opposite. "I felt my team was more tense than before the first match at Manchester," the coach said. "My team was too serious during the final training." Trying to ease the anger, PSG invited several hundreds of supporters to attend Sunday's training at the Parc des Princes. The move backfired as PSG players were greeted by insults and jeers.
"The Ultras are sad and angry," Tuchel said. "Everybody can understand that. But it's impossible to feel worse than we did." Tuchel, who has one year left on his contract, said he wants to stay at the club and help his players improve. He first wants them to react with pride against Dijon, then against Marseille on Sunday. Unlike PSG, Marseille has won the Champions League once, back in 1993.
"We have the chance to show we can react as champions," Tuchel said, with his team poised for a sixth French league title in seven years. "We are not on holiday."
More AP soccer: https://apnews.com/apf-Soccer and https://twitter.com/AP_Sports