One should never say never. Particularly in football.
Yet Paris Saint-Germain president Nasser Al-Khelaifi did it twice in the same breath last summer.
"I'll be clear: Kylian is going to stay with PSG," the club's president told L'Equipe in June 2021. "We're never going to sell him and he'll never leave for free."
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It was startlingly definitive in the circumstances, and counter-productive, too, because even Mbappe was taken aback.
The forward freely admits he found it "scary".
"When I heard [him say] that, I swallowed hard," he revealed to RMC Sport in October. "I thought, 'So what's going to happen now?'"
That is what everybody wanted to know; what everybody still wants to know.
As Paris Saint-Germain prepare to host Real Madrid in the Champions League last 16 on Tuesday, the football world is still wondering which of those two clubs Mbappe will be representing next season.
The smart money, though, is on Madrid.
It is no secret that Mbappe wants to play for the 13-time European Cup winners. As a kid, his bedroom wall was adorned with pictures of Cristiano Ronaldo in a Real Madrid jersey.
So when he talks about dreaming of wearing that famous, all-white strip, he means it literally.
He would, of course, be at Santiago Bernabeu right now if he had had his way.
Mbappe asked to leave PSG last summer. He felt his "adventure" at the club was over and that leaving for Madrid was the "logical next step".
"I wanted to discover something else," he explained. "I'd been in the French league six or seven years. I've given everything at Paris and I think I've done well.
"To arrive at 18, post-youth development, and do everything I have, I think that's something remarkable."
Madrid pushed hard to sign him last year, or at the very least conveyed the impression of pushing hard to sign him.
They offered PSG €220 million (£189m/$260m) just before the close of the transfer window – a staggering fee for a player with just under a year left on his contract – and the Madrid media machine went into overdrive.
"Mbappe, tic, tac, tic, tac" and all that.
Nonetheless, PSG remained unmoved, despite the very obvious risk of Mbappe leaving for free at the end of the 2021-22 campaign – which is what many people, including Leonardo, felt Madrid were hoping for all along.
"It seems like a strategy to try to get a no from us, to show that they have tried everything, and then wait a year to get him for free," the enraged PSG sporting director told RMC Sport in August.
"For the last two years, Real Madrid have been behaving like this and it is not correct, illegal even, because they contacted the player. It is unacceptable to us."
In the end, though, there were no legal repercussions, and it is now expected that PSG will, indeed, lose their €180m (£152m/$205m) signing to Madrid for nothing at the end of the current campaign.
The Parisians are, however, still doing what they can to try to convince him to remain at the Parc des Princes.
They have offered him a short-term contract that would make him the best-paid player in a squad that also contains Neymar and Lionel Messi.
GOAL also understands that PSG are planning to sign Ousmane Dembele when his Barcelona contract expires this summer. That would be a move not to replace Mbappe, but instead to persuade him to stay, given the two France internationals are good friends.
There has even been talk of replacing coach Mauricio Pochettino – who is expected to leave this summer no matter how the season pans out – with one of Mbappe's childhood idols, Zinedine Zidane.
The feeling in Paris, though, is that the club are fighting a losing battle, that Mbappe's mind is already made up.
Contrary to recent reports, nothing has been signed as of yet. There is no pre-contract agreement, as Mbappe has always been at pains to point out.
However, sources say that Madrid are 99 percent sure that the World Cup winner is on his way.
They have been waiting a while for him, of course. Madrid first invited Mbappe to the Bernabeu all the way back in December 2012, in the same week that he turned 14.
However, while the Spaniards were keen to sign him, Mbappe's family viewed the trip more as a treat than a trial. It was more about Mbappe meeting his heroes, Ronaldo and Zidane, than actually moving to Madrid.
"My parents wanted me to start my career in France, to be educated in France," he explained to L'Obs last June.
"To go to Spain, even if it was with Zidane, it would have been another country, another culture..."
Now, though, Mbappe is ready to go. He's always had what he calls a "career plan" and Madrid have always been a major part of it.
Florentino Perez has also been working towards this transfer for some time too, reducing the Blancos' wage bill and splashing less cash on new players in recent windows, not just because of the economic crisis caused by Covid-19, but also as a way of making room for Mbappe.
Nothing is being said in public, of course, and Mbappe has been particularly keen to play down talk of a switch to Madrid in the lead-up to this week's last-16 first leg out of respect for his current employers.
"Have I made a decision over my future? No," he insisted in an interview with Amazon Prime Video last week.
"Playing against Real Madrid changes a lot of things. Although I have the freedom to do what I want at this moment, I am not going to speak with a rival or do that type of thing.
"I am concentrating on winning against Real Madrid. And then we will see what happens."
What is likely to happen, though, is Mbappe will confirm a move to Madrid in May. Something that Al-Khelaifi said would never happen, now feels inevitable.
Tic, tac, tic, tac....
Additional reporting by Marc Mechenoua and Jorge Castro Picon