Ronaldo faced Valencia in his first Champions League match for Juventus.
The match was Valencia versus Juventus at Mestalla on 19 September 2018, during the group stage of the 2018–19 Champions League. Ronaldo had joined Juventus from Real Madrid that summer, making the fixture his first European appearance for the Italian club.
Juventus won 2–0 through two Miralem Pjanić penalties, but Ronaldo’s debut ended controversially. He was sent off in the first half after an incident involving Valencia defender Jeison Murillo. It was the first red card of Ronaldo’s Champions League career, arriving in his 154th appearance in the competition according to contemporary UEFA coverage.
The question asks whom he faced, not where the game was played or who scored. Valencia was the opponent; Mestalla was the venue. Ronaldo missed Juventus’s next Champions League match through suspension, a home game against Young Boys.