PSG won 4-1 in the first-leg of their Champions League round of 16 tie.
"They were superior and the scored goals," Griezmann said. "We had chances but it was not our best game. You need to be at your best against these type of sides. We were not today.
"It's not the image we want to give, of course. We have to work hard, fight until the final second. It was not a good game, it was a bad game, and we have to work hard and move forward."
On the return leg in Paris in March, he added: "We will go there to try and win, to get through. We know it's complicated but go we won't go there just to visit.
"Against these teams need an almost perfect game and didn't do that today. There was a moment when we were better ... but we don't like games like this. It's hard conceding four at home. Now for the second leg."