Part of the problem is having poor footballing defenders. Sure, I'm not saying we should go sign a player who absolutely can't defend, but the ability to join plays and help string intricate passes together is more important if you as me.
Marcelo is the best leftback in the world because of his footballing skills, not his defending. He is a decent defender, but a fantastic footballer. Passing, movement, crossing, dribbling, and a great ball touch are his strengths.
He'd make us immensely better, even though he is actually a weaker defender than Kolarov.