He featured intermittently across his maiden campaign, coming off the bench against Real Madrid and Barcelona as Athletic beat both rivals to claim the 2020–21 Supercopa de España in January 2021, but not selected in the squad for the 2020 Copa del Rey Final (delayed from the previous season) three months later, which ended in defeat to Real Sociedad. Having scored in the quarter-final penalty shootout and assisted the decisive goal in the semi-final of the 2020–21 Copa del Rey, Morcillo was named among the substitutes for the 2021 final which took place on 17 April, just two weeks after the previous edition; he did not get on the field and Athletic lost 4–0 to Barcelona. He scored for the first time at the end of that month, providing the opening goal in a 2–2 draw at home to Real Valladolid.