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.
"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.
And Lopetegui also hailed the performance of Modric, who was linked with a move to Serie A side Inter throughout the transfer window.
"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."
|Nadal: Victory over Ebden a step forward|
|Proteas rest Steyn and De Kock for opening ODIs|
|Nadal and Federer march on as rising stars prosper|
|Barcelona president Bartomeu convinced Valverde will stay|
|Inter forward Keita suffers thigh injury|
|Nadal races into third round with easy Ebden win|
|Kimmich ´perfect´ for Barcelona, says Xavi|
|Richardson and Nabi inspire Renegades´ rout of Sixers|
|This Liverpool isn´t a team of chokers – Mane|
|Central Coast Mariners 2 Melbourne City 1: Late Simon winner ends 10-match losing streak|
|Bryan backs Murray to return after hip resurfacing surgery|
|Scotland coach Townsend turns to trio of uncapped hookers|
|Spurs´ Aurier arrested on suspicion of assault, released without charge|
|That was a fireworks show - Warriors revel in record-breaking first quarter|
|We are in talks with Chelsea – Salihamidzic reiterates interest in Hudson-Odoi|
|´Proud´ Messi recognises impact of team-mates after reaching 400 LaLiga goals|
|Henry v Vieira: The Arsene Wenger proteges to move into coaching|
|Federer enjoys ´cat and mouse´ win over Evans|
|Wozniacki cruises through in Melbourne|
|Federer maintains perfect Australian Open second-round record|
|Warriors set NBA record, Butler´s 76ers rout Timberwolves|
|´I like the fact we beat them´ - 76ers´ Butler revels in win over Timberwolves|
|Murray says he´ll ´probably´ have operation|
|Australian Open tribute for Andy Murray surprises brother Jamie|
|Warriors score NBA record 51 points in first quarter against Nuggets|
|Panthers owner Tepper doesn´t rule out Newton missing 2019 because of shoulder injury|
|De Ligt will be world´s best centre-back – Koeman|
|Ronaldo a ´perfect machine´, says Gattuso|
|Barcelona? I can´t say anything - Stuani|
|Silva upbeat about Man City title chances|
|Celtics youngsters say Irving´s callout ´came from a good place´|
|NFL playoffs: Patrick Mahomes v Tom Brady|
|NFL playoffs: Jared Goff v Drew Brees|
|Sixers sign Brewer to 10-day contract|
|Lozano and Morgan scoop CONCACAF awards|
|Lakers ´optimistic´ about LeBron injury|
|Giants produce mammoth fourth quarter against Nymburk|
|Austin FC to join MLS in 2021|
|Henry, Vieira look forward to reunion when Monaco host Nice|
|Barcelona reject Neymar phone call reports|
|Wentz is the quarterback ´going forward´, says Eagles coach Pederson|
|Gattuso: Higuain has not asked to leave|
|Drew Brees by the Opta numbers: 10 facts for QB´s milestone birthday|
|Harry Kane injury: England captain´s history of ankle woe|
|Harry Kane injury: Opta numbers suggest Spurs struggles|
|Harry Kane injury: Tottenham´s options in striker´s absence|
|Allegri: Supercoppa clash practice for Champions League knockouts|
|First in clean sheets, second to Schmeichel in titles – Cech´s Premier League career in Opta numbers|
|Warrington sign England star Widdop|
|Gatland confident Wales can win Six Nations|
|Fulham sign Netherlands international Babel|
|Harry Kane injury: Games the Tottenham striker will miss|
|Simeone focused on matches, not transfer dealings|
|Iran v Iraq: Pressure off for Queiroz & Katanec|
|Vietnam v Yemen: Kocian keeps the faith|
|Darren Bravo back in West Indies Test side for first time since 2016|
|Kane out until March with ankle injury|
|South Korea v China: Son arrives for Group C dead rubber|
|Kyrgyzstan v Philippines: Eriksson remains hopeful|
|McGrath to miss Six Nations, Sexton nearing fitness|
|Kohli savours Dhoni ´classic´|
|Ford: People obsess over Farrell fly-half battle|
|EFL asks Leeds to explain ´spygate´ scandal|
|No alarms for Djokovic and Zverev, Raonic masters Kyrgios|
|Australia 3 Syria 2: Rogic breaks hearts with stoppage-time winner|
|Palestine 0 Jordan 0: Socceroos winner keeps hopes alive for Lions of Canaan|
|Halep: The results don´t matter any more|
|Hard work for Halep as Serena and Osaka sail through|
|Napoli director to take legal action over ´false´ quotes about Rog move|
|Alexander-Arnold facing absence of up to four weeks|