How is that? They defended all the time in the first 45 minutes and didn't concede until the 48th. With 9 men in the box they could have survived for 30-35 minutes, just like Chelsea did it in 2009, like Inter did it for so many minutes with a man less.
If they went defensive after the red card they might have eliminated Barca.
If it was Arsenals previous defense, and with Song on the pitch, and not a clown who lost the ball at every touch in Diaby, I would given them better chance with bunker defense. But Barca had waaaaaaaaaaaay to much of the ball, and all of Arsenals defenders except Sagna are too inexperienced/mediocre to handled the way Barca attacked them (though Djourou made crucial last tackles, showed his inexperience in several scenarios). Also too many offensive players on their midfield, lazy ones like Rosicky, and a very very tired Cesc who couldnt run after first half. They would needed more hardened ballwinners (who is just clears it afterwards), to have real shot in bunkering up tactic. But with that defense, and soft midfield, 2-1 one to overtime at best I think, but not more.
Also, Barca were two times better on 2nd half then they were on 1st, and thats despite fully dominating first half.