Manuel Pellegrini's side went ahead at the Cardiff City Stadium through Edin Dzeko but an effort from Aron Gunnarsson and two Fraizer Campbell headers, both from corners, turned the tide as the Welsh side won 3-2.
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"Two goals from set pieces for us is hard," said the Argentina after the match.
"It has changed for us as last season we were marking zonally and now it is individual marking and sometimes it is hard.
"But Campbell was very strong in the air. They did well defensively and closed the space well and didn’t give us the chance to create many opportunities.
"We didn't defend well at set-pieces and it cost us the game.
"We had control of the ball but we were a bit static in the first half. We were the better team but the only difference was we didn’t defend well from the set pieces and conceded silly goals."