The first leg of the affair takes place at the Bernabeu on Wednesday, before Madrid visit Leganes for the second leg next week.
Madrid landed their fourth Club World Club at the back end of 2018, but the new year has not been kind to the European champions, who have picked up just one point from their two La Liga fixtures in 2019.
Santi Cazorla scored twice for Villarreal as Los Blancos were held at Estadio de la Ceramica last week, before Real Sociedad recorded a 2-0 win at the Bernabeu on Sunday night. As a result, Santiago Solari's side sit fifth in Spain's top flight and are now 10 points off the pace being set by leaders and indeed champions Barcelona.
Madrid have not actually won the Copa del Rey since 2014, but the competition might well represent their best chance of silverware this season.
Here, Sports Mole takes a look at how Madrid could line up against Leganes.
Keylor Navas will replace Thibaut Courtois between the sticks on Wednesday night, while Nacho and Alvaro Odriozola should both come into the back four as Solari shuffles his pack for the Copa del Rey clash.
Recent arrival from Manchester City Brahim Diaz is available for the European champions, although any involvement for the young Spaniard is likely to arrive from the bench.