Arsenal nick lucky draw in remarkable comeback

Watching the first 20 minutes you can see why Barcelona are the best side in Europe, and what Arsenal need to do to match them. Barcelona pressed from the front, and won the ball back, and passed it with ease. The possession stats say it all: 71% in the first half, 65% overall.
They lined up in a 4-3-3 that could switch to a 4-2-4; Keita started on the left of the centre midfield, Pedro on the right wing, and Messi just off Ibrahimovic. Both William Gallas and Cesc Fabregas started, with Arshavin and Nasri switching flanks because of Nasri's greater defensive qualities.
Almunia made some great saves and Barcelona were unlucky to go into the break at half time at 0-0. Meanwhile, Arsenal had already made 2 subs due to injury-- Gallas going off with a calf injury which could be him for the season, and Arshavin also with a calf injury. Eboue came on for Arshavin, and Eboue's more defensive qualities helped Arsenal defensively, as Eboue would track back, and prevent Barca's fullbacks from pressing, unlike Arshavin. In fact, Barcelona didn't really have a great chance after Eboue came on, and then after half time, Arsenal's defense pushed up for a higher line, which squeezed the space but also lead them susceptible to the long ball, which is how their two goals arrived. The first, a mere 24 seconds into the second half was a bit of a mistake from Almunia, off his line, but it was a good finish from Ibrahimovic. The second was a mistake from Vermaelen who pushed forward to try and catch Ibrahimovic offside, but he was easily onside, and made it 2-0. But those were the only chances Barca had in the second half, and if you look back at the 2 goals they conceded, there was no pressure on the man with the ball at all;
As this picture shows, right when Pique kicks the ball, Bendtner is still a good couple of yards off of him, and for the second goal
It was almost a carbon copy of defending for the second goal, Abou Diaby doesn't press Xavi, and he ahs all the time in the world to play the perfect ball.
Walcott came on in an inspired sub, and was the difference. Arsenal had pace to hit Barca on the counter, and unsettled Maxwell. Walcott is at his best as an impact sub, and scored after Bendtner played him in. It was Walcott's cross that lead to the penalty that Cesc put away, though damaging his leg in the process. A great comeback, but possibly a costly one.