Right-back: Ferland Mendy (Real Madrid)
Real Madrid have some difficulties in this position, with Carvajal out injured, Odriozola on loan to Bayern, and Nacho looking out of depth. This prompted Zidane to play Mendy as a right-back against Shakhtar, and he too had a stinker. However, we've seen better from the Frenchman, and his expansive skill set could see him start here again on Saturday.
Centre-back: Gerard Pique (Barcelona)
While it's no secret that Pique is no longer the defensive force he used to be, the 33-year-old continues to have a huge influence on the squad. He brings a wealth of experience and big-game pedigree to the backline, which could prove decisive in the Clasico, a fixture in which he often comes up big for Barcelona.
Centre-back: Sergio Ramos (Real Madrid)
Unlike his Clasico rival and compatriot, Sergio Ramos still rules the roost when it comes to defending. He's not just a great leader, but also an excellent reader of the game, turning up at the right place at the right time to deny the opposite sides with crucial blocks and interceptions. Apart from defending, he also brings his penalty-kick prowess.
Left-back: Sergino Dest (Barcelona)
With Jordi Alba still not fully recovered from injury and Marcelo a spent force, Barca's new signing gets the nod here. Sergino Dest filled in for the Spaniard against Ferencvaros and might do so again on Saturday. He showed great attacking intent, driving forward to link up with Fati, though he's got to be wary of his movements against Madrid's marauding wingers.