Ref didn't even give Arsenal the chance to regroup. 
He urged Fabianski to return the ball, then so generously handed
the ball directly to the Porto's player's hands...Fabianski haven't
even walk back to his goal line.
I think the Arsenal players only have themselves to blame... They
were slow in getting back. Vermaelen was closest and yet he was
seen slowly walking back and still pointing to the
sky... Falcao was still no where to be seen when he was
already at the box... Vermaelen could have prevented it or made it
more difficult for Falcao but he switched off and let that guy ran
pass him...