Rakitic is a very good player... he plays well and helps the squad a lot. After all, he is Sevilla's captain this season and I admire him, his character and skills, especially his long balls, corner kicks and free kicks (crosses in the box, as he rarely shoots from a free kick). Nevertheless, I can't imagine him replacing Gabi at Atletico. Gabi doesn't seem multi-dimensional and a talented player, or at least he doesn't give one an impression as such. However, Rakitic will never ever in his career even come close to Gabi's work rate during a match. Gabi's pressure, running, marking and fight could not be compared to Rak's, and that is what Rakitic's role actually is most of the time, the DM-CM thing... but he just doesn't do it. He has his sparks, like against Madrid, putting Pepe in shame, but I guess you have seen his first half performance = disastrous. That is Rakitic in most of the matches, in both halves of the game, slow and lazy, but still has those sparks. Would that be enough in Atletico's powerful and winner's machinery ? Probably not.
I fear that if Captain Rak would sign for Atletico, he will not be more than a sub... unless they change their playing style and somehow make way for Rak to slide into the first team. I doubt that very much, considering that Koke and/or Suarez, Thiago are also needed. The only way Rak could play as a first-teamer in Atletico is if they would feature both him and Gabi at the same time, which could happen, but I still doubt since they more or less have the same role. It would mean for one of them to be forced to play more deep or more forward upfront, and that would possibly not turn out to be effective.