The Portuguese can expect a hostile reception in Valencia, who had to suffer 15 of his goals for Real Madrid and are returning to the Champions League for the first time in three years.
The week's other mouth-watering game is last season's runners-up Liverpool hosting French champions Paris St Germain, while Real Madrid begin their bid to win a fourth straight Champions League title at home to AS Roma.
English champions Manchester City host Olympique Lyonnais and five-times European champions Barcelona host Dutch league winners PSV Eindhoven.
Fresh from scoring his first goals for Juventus, Cristiano Ronaldo makes an eagerly-awaited return to Spain when the Italian champions play Valencia on Wednesday in their opening UEFA Champions League group game.
All eyes will be on the Portuguese forward, the all-time top scorer in Europe's elite competition and a five-times winner of the trophy, when he makes his first public appearance in Spain since ending a trophy-filled nine-year stay at Real in July.
Ronaldo's last act for Real was lifting the Champions League trophy in Kiev after beating Liverpool 3-1 in the final before he dropped the bombshell in a pitch-side interview that he had decided to leave Madrid.
Ronaldo can expect a hostile reception in Valencia, who had to suffer 15 of his goals for Real and are returning to the Champions League for the first time in three years, hoping to improve on their miserable start to the La Liga campaign.
Ronaldo, who plundered a staggering 451 goals in 438 games for Real, made a slow start by his usual standards to life at Juventus by failing to score in his first three games but broke his duck with two strikes in Sunday's 2-1 win over Sassuolo.
"I really wanted to score these first goals and I’m happy to have found the net," Ronaldo told reporters.
"Obviously, I was a little anxious with all the talk after my move from Real Madrid and not scoring and there was a lot of expectation on me, but I knew I was working hard and it was only a matter of time before I scored."
Juventus, knocked out in the quarter-finals of last season's competition by a Ronaldo-inspired Real Madrid, will be looking to make a strong start in Group H, which also contains Swiss champions Young Boys and Manchester United, where Ronaldo became a household name before joining Real in 2009.
Real begin their bid to win a fourth straight Champions League title at home to AS Roma on Wednesday after drawing 1-1 at Athletic Bilbao, where they certainly could have done with the firepower of all-time top scorer Ronaldo.
The week's other mouth-watering game is last season's runners-up Liverpool hosting French champions Paris St Germain, who begin another quest to win Europe's elite competition for the first time and justify the massive investment ploughed into the club by their Qatari owners since 2011.
Both sides have made perfect starts to their domestic campaigns with five wins, PSG thrashing St Etienne 4-0 at home on Friday and Liverpool enjoying a convincing 2-1 victory at Tottenham Hotspur.
English champions Manchester City host Olympique Lyonnais in Group F on Wednesday also looking to make a mark in the competition they are yet to win despite huge investment from their Abu Dhabi owners.
Five-times European champions Barcelona host Dutch league winners PSV Eindhoven in Group B on Tuesday looking to improve on their recent disappointing showings in the competition, having been knocked out at the quarter-final stage in the last three seasons.
