As much as the dearly departed Meta would love to tell me otherwise, Mascherano is not a natural CB, never will be a natural CB.
However, for Barca's system, he is very good in the CB role assuming he's playing on a legitimate backline. He doesn't have the natural sense to cover up when he's around someone as lost in the role as Song is, and without structure, he too will be lost.
It's simple, the fault is not the players who are mostly excellent when used properly, the fault is Zubi or Tito or Pep or whoever it was who insisted that Masch, Piquez and a Puyol near the end are enough.
Mascherano is great next to Puyol or Pique. But the two of them have been injury prone for a while now, and in Puyol's case the clock is ticking.
I though people who point out Masch's flaws are haters?
He shouldn't have been sent off, but the tackle that led to the goal sums up Masch's defending for me. If you're gonna commit to a challenge in a place like that either make sure you get it 100% spot on or don't bother, there's no in between. Often a clinical tackler but more often caught hesitating and in no mans land. He needs to be a lot more decisive, focused and clinical to be considered a world class CB for me.
The thing is he effectively plays like a DM still, albeit a very, very deep one.