Marcelo is the best LB in the game imo but in my books Sandro is holding the second place. I'd even rotate them sometimes, or push one more forward. But hell, if Marcelo is injured then there shouldn't even be any doubt about him starting while nobody else steps on the left back position.
Well, it is when he's been left out all this time. Marcelo and Filipe Silva have been Tite's choices so far, so I'm happy this guy might finally get the chance to start, maybe impress and at least become vice-Marcelo.