Rakitic is doing well in certain conditions- basically with bunkered defences and Messi playing right wing.
1. Working with Messi and Alves to provide overload on the right.
2. Has some space facing goal a lot of the time, doesn't have to make as many quick decisions.
Of course, his speed defensively is much better than Xavi. And Xavi is superfluous in games where the opponent retreats quickly into double banks and there is no need for midfield control. Rakitic is a better choice under these conditions, and it may well be the majority of games that opponents play a bunker/counter tactic.
He needs to improve his scanning skills. He doesn't know what's outside his field of view*, so when receiving a pass with his back to goal, he needs some space and time to release. This is why he appeared a bit lost in other games, where there was more pressure on the midfield. If he can learn to be a bit more Xavi/Modric in his awareness, he'd be even more valuable.
*He does see counter options well during transitions and releases the pass early. I'm referring to his play with Barça in possession.