I don't know why people blame ONLY Arda and Arda's sub.
For me, the worst thing is that Lucho doesn't make a distinction between Cms and Cams.
He starts with Rakitic and Gomes, and end a game with 3 Cams in Iniesta, Denis and Arda.
Arda made a mistake, but Lucho started a game with two Cms who are more defensive and less creative (Rakitic and Gomes), and ends it with 3 guys who can't defend.
2 minutes before a 1:1, Denis was easily fooled by Marcelo when Cr7 almost headed a ball into net.
Lucho probably subbed in guys who can run since they are fresh (his logic probably, which is ok to some extent).
But neither of those 3 can defend too much.
Not to mention that we lost an arial strength on defending set pieces without Rakitic and Gomes, and with short and tiny: Denis and Iniesta, plus somewhat taller Arda.
** During that free kick, we had only 2 taller players on a pitch: Pique and Busi.
And: Messi, Iniesta, Denis, Alba, Roberto, Masch, Arda (and Luis, he is ok).