Cristiano Ronaldo is the reason why most people expect Juventus to progress past Porto in Tuesday's Champions League last-16 second leg in Turin.
But there are also those who believe he is the reason why the Bianconeri have a 2-1 deficit to overcome.
Juve were abysmal in the first leg at the Dragao, and Ronaldo unusually ineffective. As the Gazzetta dello Sport pointed out in their match ratings, the Portuguese had Rodrigo Bentancur to thank for avoiding the ignominy of being labelled the worst player on the pitch.
Ten days later, Juve were held to a 1-1 draw at Verona, a result which left Andrea Pirlo's side 10 points off the top of the Serie A standings.
Ronaldo netted at the Marcantonio Bentegodi, but his worth was nonetheless called into question by Antonio Cassano.
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"Juventus signed him to win the Champions League, but they have had worse results since he arrived," the retired forward told the Corriere dello Sport, alluding to the fact that a club that made two finals in three years between 2015 and 2017 has failed to even reach the semis for the past two seasons.
"They would have won Serie A titles without him anyway. It was the wrong project... Ronaldo has nothing to do with Pirlo's ideas. He will carry on scoring because he can even do it while sitting down.
"But Andrea wants to build the action from the back, he wants to press high on the pitch. Ronaldo can decide games, but he doesn’t participate in them much.
"I think he will have done badly at Juve unless they manage to win the Champions League."
Firstly, it is worth bearing in mind that Cassano is an outspoken character who courted controversy throughout a career that failed to live up to early expectations, so Pirlo certainly will not be questioning Ronaldo's role within his team just because his former Italy team-mate has spoken out.
As the Juve boss dismissively wrote of Cassano in his autobiography, "Antonio claims to have slept with over 700 women, but he's no longer called by the national team, so can he be happy?"
But are Juve happy with Ronaldo? And, more pertinently, is Ronaldo happy with Juve? Or is a love affair that began while the Portuguese was still a Real Madrid player now destined to end in divorce?
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The Bianconeri never made any secret of the fact that they bought the Blancos' all-time top scorer for two reasons: to win them the Champions League; and boost their commercial appeal.
As we enter the third year of Ronaldo's four-year contract, only one of those objectives has been achieved.
Before his €100 million (£86m/$119m) signing, Juve had approximately 49 million followers across their social media channels – that figure has now more than doubled (102.9m), and the resulting boom has, unsurprisingly, made the club far more attractive to investors.
In January 2019, the Bianconeri signed a new, improved, seven-year sponsorship deal with their kit manufacturer, adidas, worth €357m (£308m/$395m) – twice the value of the previous agreement – and that partnership has since been extended until 2026-27.
In addition, Juve have benefited from a €25m (£21m/$30m) increase in the annual value of their contract with shirt sponsors JEEP. Ronaldo's influence is underlined by the fact that, before he joined, the American car manufacturer was paying the club just €17m (£15m/$19m) a year for the right to display their name on one of the most famous kits in football.
However, there is now one major problem, a problem which has been exacerbated by the economic crisis caused by Covid-19: Ronaldo's salary, which accounts for €31m (£27m/$37m) of Juve's €236m (£203m/$281) wage bill.
No elite European club has been hit harder by having to play games behind closed doors than Juve, who experienced a 36 per cent drop in matchday income in 2019-20, according to the latest Deloitte Football Money League report.
The situation has not improved in the interim either. Last month, Juve published their accounts for the first six months of the current campaign, revealing a loss of €113.7m (£98m/$135m).
The club also admitted that they could "dispose of players' registration rights" at the end of the summer in order to ease their financial concerns.
Offloading Paulo Dybala would certainly make sense, given his market value, fitness issues and inability to nail down a regular starting place. However, selling Ronaldo to the highest bidder would make even more sense.
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Ronaldo costs Juve around €85m (£73m/$94m) a year in wages, taxes and amortisation (the money the club has committed to paying on an annual basis to cover the striker's transfer fee over the four years of his contract).
That contract expires in the summer of 2022, so if Juve are to make any money back on his exit, they will have to sell him at the conclusion of the current campaign.
What's more, Ronaldo may be willing to leave too.
He has become an expert in defying the ravages of time but, at 36, he will not have many more shots at Champions League and Ballon d'Or glory.
The most pertinent issue, then, is whether he truly believes he can lift either trophy again as a Juventus player because, as it stands, the Bianconeri will struggle to win Serie A this season – let alone the Champions League.
Fabio Capello has accused Juve of "playing rugby" under Pirlo on account of all of their lateral passing, while at the same time acknowledging that the coach's attempts to introduce a daring new footballing philosophy have been seriously hindered by a historically congested calendar that has reduced preparation time but increased injuries.
It has certainly been noteworthy how much more effective Juve have looked with the fit-again Alvaro Morata leading the line, with the Spaniard proving decisive in last week's wins over Spezia and Lazio, which have drawn Pirlo's side to within seven points of Serie A leaders Inter.
Nonetheless, his fluid formation and team selections have come under intense scrutiny of late, resulting in the normally placid Pirlo snapping at journalists with greater regularity in recent weeks.
Sporting director Fabio Paratici insists that the club retains full faith in the 41-year-old, but, one wonders, does Ronaldo?
Indeed, if Juve stick with Pirlo beyond the summer, Ronaldo will be forced to evaluate whether he can really afford to spend another year spearheading a relatively young team still finding their way under a coaching novice.
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If Juve decide they need a more experienced tactician to deal with an unprecedented crisis, Ronaldo would be facing the prospect of a fourth season in Turin under a fourth different boss.
Neither scenario will feel particularly appealing to the forward right now and frustration would be an understandable reaction to the situation he now finds himself in.
After all, he has undeniably held up his side of the bargain, both on and off the field, scoring 92 times in just 119 appearances for a club that has benefited enormously from being associated with the 'CR7' brand.
He would be well within his rights to argue that it is not his fault that Juve are no closer to winning the Champions League than when he arrived.
However, Cassano and others are also entitled to claim that his exorbitant pay packet has prevented the club from strengthening an imbalanced squad, particularly in midfield.
Juve would have dearly loved to have signed either Paul Pogba or Sergej Milinkovic-Savic last summer, but they simply did not have the resources to do so. The pandemic was largely responsible for their financial constraints, but Ronaldo's wages also undeniably played a part.
Remember, committing €340m (£292m/$404m) in transfer fees and wages to acquire a 33-year-old attacker was always a gamble on Juve's part. They could not have predicted the pandemic, but it is worth noting that they were still losing money before it began, posting a €40m (£34m/$48m) loss at the end of the superstar's first season in Serie A.
The Ronaldo deal was a high-wire balancing act and Juve must now evaluate whether keeping him for another year is worth the risk of losing him for nothing in 2022.
Victory over Porto would undoubtedly alleviate the tension in Turin but, given Juve's obvious issues on and off the field, it would merely postpone what promises to be an awkward conversation over the future.
Like it or not, Cassano has merely publicly expressed what many have been privately pondering for some time now: Is Ronaldo holding Juve back – or are Juve holding Ronaldo back?
Sadly, it could well be a bit of both, meaning a summer separation may be the most sensible option.