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.
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.
"His time had come and he made the most of it," Lopetegui told Movistar after his first Champions League match in charge of Madrid, winners of the tournament three years in a row.
"I'll take the team's triumph against a rival like Roma, who last year eliminated Barcelona and went far [the semi-finals].
"We knew they were going to make things difficult for us, but the team was very good, we made a great match, both defensively and offensively."
Luka Modric shone in midfield, setting up Bale for his goal, with Isco calling for the Croatia star to be given the 2018 Ballon d'Or in a post-match interview.
"He has played a great game," the former Spain coach said. "He just missed a goal.
"You know that if you do not take advantage of the opportunities sooner or later the opponent can have them.
"But the team has played a great game and has deserved the three points."