Centre-back: Eder Militao (Real Madrid)
Real Madrid's Eder Militao stepped up to the plate in the absence of Sergio Ramos and Raphael Varane last season, proving to be an excellent deputy. Now, with both of Madrid's old guards gone, he has become more crucial in their set-up.
He cleans up danger with his excellent man-marking and vision, and is no slouch on the ball himself. Militao has completed 90.8% of his passes this season, which is among the best in La Liga. He has also averaged 2.6 clearances per game, and is strong in the air too.
Centre-back: Gerard Pique (Barcelona)
Gerard Pique may be long past his prime, but the fact he's still at Barcelona is a testament to his longevity and value at the club.
He's still assertive in one-on-one situations, exudes confidence on the ball, and is dominant in the air. His winner against Dynamo Kyiv on Tuesday made him the joint-most prolific goalscoring defender in Champions League history.
Pique is now also the oldest Barcelona player to score in the continental competition. He will look to continue his good form against Real Madrid on Sunday as Barcelona seek to close the gap on the league leaders.
Centre-back: David Alaba (Real Madrid)
In a short while, David Alaba has established himself as a key figure at Real Madrid. Arriving from Bayern Munich on a Bosman, the Austrian has looked right at home at the heart of their defence.
Incredibly versatile, Alaba offers Carlo Ancelotti immense tactical flexibility. His leadership qualities have also filled the void left by Ramos. One of his greatest assets is passing; he currently averages 91.3% in La Liga, and has registered two assists too.