Well since he's going to be starting instead of Mascherano, I believe he CAN be an improvement over him. He is faster and will be infinitely better in the air, which gives us a dimension we often get exposed for in defense. I just pray that he is back there to defend those aerial duels rather than sitting way up in la-la land
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We don't need a CB Alves. He can be involved in our buildup (he'll probably be much more involved in that aspect than Pique) but he can't break the stability of our defensive line by wandering around out of position. Even if our defenders aren't particularly great, if we can maintain a protected and disciplined defensive line than we won't be easy to break down even if we don't have world class defenders.
This is where I feel Lucho will come into play. Usually, Busi's disciplined positioning in front of our CB's gives us a defensive shape and creates a structure that cuts off a lot of space for attackers. This season he (along with the likes of Alves) roams around far too much and we've seen a lot less of defensive Bossquets than previous years providing a backbone. Lucho likes to keep a "Busquets figure" in his midfield and whoever it is (Oubina or Fontas) remains disciplined and helps keep a balanced defensive structure. Anytime Celta's defensive shape breaks it's due to individual error instead of having an unorganized and exposed defense from the start (reasonable due to lack of ball playing ability in Celta's defense and no world class players). I hope that because of this that Lucho is able to get the most out of our defense and mask Luiz's faults so that his qualities shine and dominate (much like Ancelotti has done for Ramos and Pepe at Madrid).