Before this season Marcelo was consensus best LB right? More or less I think. It's hard to tell if his dip in form is because of his team, decline or just lack of motivation. I don't see his attacking skill declining. In that respect he reminds me of Alves where if you have a lot of possession, you can count on this guy supplying something.
If you are relying on this guy to defend then you're going to have a bad time. But what big team is really hoping their LB defends everyone? They are hoping to have the ball.
If we are planning a huge mercato, a big coaching change and a really, really serious team next year, then I would swap this guy in for Alex Sandro. I think we can come out +/- around even in that scenario, money wise (wages would be higher).
If we are just going to buy Marcelo and swap around the deck chairs in midfield, then I would pass.