Mariano Diaz took his chance by scoring on his first appearance since returning to Real Madrid, said head coach Julen Lopetegui.
The striker returned to Madrid after a single season in Ligue 1 with Lyon and Lopetegui had to explain why he left Mariano on the bench as his side stumbled to a 1-1 LaLiga draw at Athletic Bilbao on Saturday.
Isco and Gareth Bale had already put Madrid 2-0 up against Roma in Wednesday's Group G opener when Mariano replaced the Wales star with 17 minutes to play.
And Mariano made the most of his cameo opportunity with a brilliant finish into the top-right corner that gave the scoreline the emphatic look Madrid's dominance deserved.