He's just not up to the standard of Alonso or Mascherano.
And I don't think he will be.
Well he is definitely not the standard of Alonso but then again Alonso shares a group with the likes of Xavi, Cesc, and then im struggling to find peers.
As great as Xabi Alonso was, his last season here set an impossible standard for any CM to follow. Xabi in the two previous seasons was nowhere near his form of 08/09. He was getting caught in possession, he was slow, and it looked like if you pressured him you could disrupt his game. Thats when Rafa decided to move to a more mobile, dynamic midfielder who could affect the game in more advanced areas in stead of relying on deep lying positions.
After that tremendous kick up the ass, Xabi produced world class performances week in and week out. It was remarkable.
So if you are expecting a 23 yr old lucas to produce the class of a 27 year old alonso then I think you are being very unreasonable.
As far Mascherano, Lucas is more valuable because he isn't so limited. Mascherano is probably the best defensive midfielder in the world which is all well in good but if you are gonna play in midfield you have to be able to contribute something to the attack which he provides nothing. His distribution is side to side without thought, he has no incisiveness to his game whatsoever and does not support attacks. Not to mention his horrendous shooting ability which rears its head every few matches to ill effect.
Mascherano is only valuable for the toughest fixtures and even then Lucas has shown against Chelsea that he can do the defensive job well too. He dominated that game with his tackling and passing. Mascherano does not have that ability on both ends. Its why hes not playing in barca's midfield right now. Barca bought him because they can afford it. he is a luxury that will be trotted out in the CL and spanish cup and the occasional league game.
You have to give lucas time. He is improving every year and is even improving from game to game this year.
I suspect you will only notice his quality once he reaches top class level and has his fingerprints all over games like alonso, xavi, and cesc. Otherwise he'll just be in the background to you like busquets.