Kieta most certainly hurt the attack. I don't see the reason for taking Villa off either when Arsenal obviously where starting to get caught forward. However, if you want to kill the game it would have been easy to slot Masch in behind Sergi and in front of the defense to cut off those quick outlet passes (which is/was Arsenal's only hope) and let Sergi get involved with a ball a little more to provide for better possession.
Furthermore, I don't know why Barca became so damn narrow in the second half and the reason is this.. Arsenal were getting ripped by one simple final pass between or right around their CB's for the whole of the first half; ALL of Barca's chances in the first were created from that simple pass. Wenger did well to adapt and he bunched the middle of the field up so as to cover that space but in doing so he let his CM's sit very narrow and let his fullbacks pinch in leaving acres of space behind and to the sideline of each fullback.
However, Barca continued to try to force passes through the middle and lost a lot more balls than usual during the second half. A wide-run from Maxwell rewarded by a diagonal ball would have easily given Barca a chance on goal, or if not anything else, then a couple of these passes would have forced Arsenal to open the center of their last two lines up a little.