There is also a continuing political backdrop as to whether it is time to cash in and ease him out for others, such as Gareth Bale. But that importance gained a boost by the team's failure to score without him in Manchester last Tuesday, with the focus on Bale, in that turgid 0-0 draw.
Earlier this week the Spanish sports newspaper Marca led with a story headlined: "El Canon Ya Esta Listo" (the cannon is ready) and with a close-up photograph of fit-again Ronaldo's right leg to emphasise his status and his incredible goalscoring feats so far in this competition. Ronaldo is already the Champions League top-scorer of all time with 93 goals - 10 ahead of Lionel Messi - with an astonishing 16 of those coming this season alone.
The Portuguese has played in 10 matches and scored in seven of them - with four goals against Malmo and a hat-trick against Shakhtar Donetsk and then another in the dramatic quarter-final turnaround as he scored three times at the Bernabéu to reverse the 2-0 first-leg deficit to Wolfsburg.
To put that in another context, City have scored 63 goals as a team since they made their first appearance in the Champions League in September 2011. Ronaldo has scored 65 goals on his own since then.
"I don't doubt that I will go down in footballing history," Ronaldo said. "Whether people like it or not the numbers speak for themselves. I will be up there with the rest. Some like it more, some like it less, but I have no doubt that I'm already in the history of football."
Although that is undoubtedly true, it is a fascinating quote as it speaks to Ronaldo's desire for individual, immortal recognition, to chasing the "numbers" to achieve that and to do so in what is a team game. There is also an acknowledgement that he is not universally popular.
And it speaks to something else. Should Real beat City and then triumph in the final in Milan Ronaldo will want to have made a significant contribution. For all those goals, for the fact that he has two Champions League winners' medals already - one with Real and another with Manchester United - and has scored in both those finals there is not a final that he can call his own.
In 2008, when United beat Chelsea on penalties after a 1-1 draw, Ronaldo scored but missed in the shoot-out. In 2009 United were overwhelmed by Barcelona and Ronaldo was eclipsed by Messi and then even in 2014 when Real beat Atlético Madrid 4-1 after extra-time, it was a final that owed more to the contributions of Bale and, in particular, Ángel di María.
So Ronaldo has the motivation and the pressure to know that anything short of reaching the final would throw Real into crisis. Again. "It's going to be a very difficult game," Zidane said of the semi-final second leg after declaring that Ronaldo was fit to play after missing the last three matches (although that has been claimed before without proving to be true).
"If we don't go through it will be a disaster because we want to make the final ... it won't be like the Wolfsburg match - we won't score two goals in 15 minutes. We know that we're going to suffer from the first minute to the last and the important thing is that we are prepared."
It will be a disaster also to Zidane's prospects of remaining as Real's coach and may push Ronaldo further towards the exit door - whatever his goals - if PSG or United meet the asking price and the incredible personal wage demands. Zidane's relationship with Ronaldo is far better than that of his predecessor, Rafael Benítez, who attempted to operate more of a meritocracy.
"Cristiano is not at risk to play, he is 100 per cent ... Cristiano is a different player and his numbers prove it," Zidane said while Ronaldo was eating. There will be more appetite to add to those numbers against City.
It remains to be seen whether Ronaldo plays against City. Real have put up smokescreens before and there were suggestions that he may have suffered a second muscle injury although he was certainly smiling as he sat with Mendes over their meal. "It's been a week and bit by bit he has recovered," Zidane said.
As for Ronaldo himself he claimed he would give everything. "I've been lucky enough to win it twice and obviously I want to win it more - hopefully again this year," he said. "Fans love the Champions League and obviously I love it, because it's a special competition.
"I have always worked hard, believed in my potential - in the academy at Sporting, at (United) and at (Real) as well - and I've developed more and more as a player, as a person, as a human being. I've enjoyed my work."
- Telegraph