Filling in for the suspended Suárez, Alcácer played the full 90 minutes in the domestic cup final, scoring Barcelona's last goal in the 3–1 defeat of Alavés. On 5 November 2017, he netted a brace in a 2–1 home win against Sevilla that kept his team four points clear at the top of the league table. He scored his first goal for the club in the UEFA Champions League one month later, helping to a 2–0 group stage victory over Sporting CP.