Achraf, 18, was promoted to the seniors at the start of the current season after impressing during 2016-17 for the Castilla feeder team.
And with Dani Carvajal playing no part in Sunday's fixture due to a virus, Zinedine Zidane has turned to the youngster to plug a gap in his depleted defence.
Marcelo and Theo Hernandez are both also unavailable for the Blancos, meaning Nacho continues on the left side of defence.
Further forward the team takes on a more familiar look, with Zidane's first-choice midfield of Luka Modric, Casemiro and Toni Kroos taking the field.
But with Gareth Bale joining Karim Benzema on the injury list, Isco and Marco Asensio have been charged with accompanying Cristiano Ronaldo in attack.
Real Madrid: Navas; Achraf, Varane, Ramos, Nacho; Modric, Casemiro, Kroos; Isco, Ronaldo, Asensio.
Substitutes: Casilla, Vallejo, Lucas Vazquez, Llorente, Mayoral, Ceballos, Tejero.
Espanyol: Pau Lopez, Aaron, Gerard, Jurado, Leo Baptistao, David Lopez, Javi Fuego, Roca, Hermoso, Sanchez, Darder.
Substitutes: Diego Lopez, Navarro, Naldo, Garcia, Vila, Diop, Garnero.