Not a useful comparison. Xavi isn't tactically being asked to play differently (*recently; earlier on, when Lucho was experimenting, Xavi was played in some bizarre ways). Xavi still plays the metronome. Iniesta clearly has been asked to play a more withdrawn role and let the forwards have the ball.
Iniesta depends more on Xavi/Busquets than vice versa. Xavi/Busquets provided the protection, connectivity, and distribution. Iniesta, playing withdrawn and with fewer connectivity options, no longer has the opportunities to work his attacking magic as often as before. Maybe it will improve, as we saw yesterday, when Neymar attempted to be more of a connective player; if Neymar continues to do so, and refines his passing, Iniesta will become more involved in a more advanced position. If Neymar goes back to endless ball-holding, Iniesta can grab a cup of tea and take a nap until possession is turned over. I can't count the number of times this season I've watched Iniesta fruitlessly present himself for a pass from Neymar that never comes. Frustrates me, but must *really* frustrate Iniesta, seeing his chances to contribute to the game ignored.
Wouldn't say he was poor in the World Cup. The midfield was successfully bypassed by Netherlands (and, if I recall, Alonso was awful).