"You could see when you have a big difference after the first game it is hard to turn up with the same urgency than if the difference is very small and that played a part in the first half, " he said.
"They scored the second and Ignashevich had a chance to score goal three, then we were a bit on the ropes".
They are the first Austrian side to reach a European semi-final since 1996 and are now unbeaten in 25 home games in all competitions this season. Their second was from distance, but in the box we didn't give anything away.
"I can say that, until 75 minutes of the match, we played extremely well and we were winning in a very justified way", he said. "Once we scored the 2-1, it was a big blow for them, and they didn't recover from that".
But with the likes of Atletico Madrid and Olympique de Marseille in Friday's draw, Koscielny said Arsenal can ill afford to take their foot off the gas as they appeared to do in a lacklustre opening hour in Moscow. They played well, had opportunities and took them. It put pressure on us - if they score we are out. We need to be focused.
"It is our last chance to qualify for the Champions League, so we will fight until the end".
In a week that has seen Roma shock Barcelona and Juventus nearly beat Real Madrid in remarkable UEFA Champions League quarter-final matches, much of the pre-match talk in Russian Federation focused on the need for Arsenal to put in a professional performance to defend their 4-1 first-leg advantage.
It is Diego Simeone's Atletico, though, who Wenger would prefer to avoid in his side's first last-four tie in Europe since 2009.
EPS for Encana Corporation (ECA) Expected At $0.17
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