Don't you think, like, you know, Mascherano kinda knew this prior to signing for Barcelona? -_-
Mascherano is a new player, from a different league, from a different background, from a different system. He plays at the position currently occupied by one of the hottest prospects on that position, Mister Sergio Busquets. This player has been reshaped from a striker or something to that position in the youth ranks of Barcelona, there being indoctrinated by Barcelona's way of the game. The result of that combination? The man was a key factor in the world's biggest finals in football (Champions League + World Cup), which were both won, in his first two years of top class football. There is no way thinkable Mascherano would walk into the team and dismiss Busquets to the bench. Mascherano needs to adapt, learn the system, grow into the team. You know, like Barcelona does with all their cantera players: that is exactly the reason players like Busquets, Pedro, ... can make it in their first year with the A-team: they know everything and are able to play along, they are already adapted to the distinct way of playing. Mascherano is new, doesn't know the club or anyone in it (besides the argies), he will see a lot of the bench, he will learn a lot on training, he will have to take his chances when he gets playig time and he will work his way up the ladder. That's how it should go, he will have to earn it's place not by transfer, but by working hard and proving he is the best (and he cannot be now yet for the above reasons)
This is of course different than Yaya's case. Yaya was here PRIOR to Sergio Busquets. Yaya grew at Barcelona to that famous player, Yaya played keyroles in that treble season, Yaya was the man and the man had a firm grip on his position (that season Sergio was still just a substitute for Yaya). Until little Sergio breaks through and ousts him from that permanent entitlement. Guardiola recognises the talent and potential of Sergio (and more importantly: brought him with him via the youth team which means a special bond prior to the A-team relation, key fact in my opinion) and starts to rotate him and Yaya (Sergio mainly league/Yaya mainly CL) ... Yaya doesn't understand. The new kid on the block with his play-acting etc. nicks the place of the proven powerhouse? He's confused, doesn't take his chances when he is playing, his agent doesn't get it either, Yaya is off to the ACN and when back, Guardiola needed to make his choice for the season's money time and he picked Busquets as the main man with Yaya getting a more limited schedule ... and you know the rest of the story.
It's like the baby and the dog-thing. You buy a baby prior to a dog, not vice versa. Otherwise there is a big chance that the dog can't process the altering situation of priorities and attention. Not saying Yaya or Mascherano are the euivalents of dogs, but Busquets surely is Guardiola's baby.