The France international says the Gunners have nothing to lose at Camp Nou on Wednesday but admits it will be a tough ask to knock the holders out of the competition
The Premier League side face the daunting prospect of trying to overturn that deficit at Camp Nou on Wednesday, a task many believe will prove beyond their capabilities.
Since losing the first leg on February 23 Arsenal have only won once in all competitions, a run which has seen them fall 11 points behind in the title race and suffer elimination from the FA Cup.
Progressing in Europe looks unlikely too, but Koscielny still believes they could cause an upset against Luis Enrique's side.
"We don't have pressure like in the first game, because we play against the best team in the world," he told Arsenal Player.
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"Now we need to play the second leg away at Barca and we have nothing to lose. People always talk about the statistics and I think we have maybe a five per cent chance to qualify.
"You know when you play against the best players in the world, the best team with a good collective side, it's very important for us. It can be very important if we can have a [good] result.
"They have a lot of quality, they can have a lot of possession during the game and when they accelerate with their top players, they create chances.
"We need to play with freedom and believe in our quality because we did well during the first six months of this season. I don't see why we can't [play like that] against Barcelona and for the rest of the season.
"You know we are in a difficult period and we need to stay like a unit to be stronger. We will fight until the end of the season."