Arsenal goalkeeper Wojciech Szczesny admits their performance at Borussia Dortmund was "not good enough".
The Gunners lost their opening Champions League game 2-0.
The Pole told arsenal.com: "It wasn't good. We lost two goals and it was probably lucky that we didn't concede more. They counter-attacked us very well. On another day they probably could have finished more [chances] so it was very disappointing.
"We didn't do it well enough so now we need to improve for the second game."
