The boy who plastered posters of Ronaldo onto his bedroom walls had been applauded by Real Madrid's fans when PSG played at the Santiago Bernabeu in the Champions League in March.
There was even a degree of appreciation for his two goals, which left Madrid on the brink of defeat. This was at least a preview, they thought, of the player that would soon belong to them.
Those expectations have been swelling for years, the protracted nature of this saga creating a longing at Real Madrid and a fanfare in the media.
The hype made Mbappe joining Madrid feel both inevitable and, at the same time, absolutely essential.
The club have been obsessed by Mbappe. The pandemic restricted spending but there has also been method in their restraint, a building up of funds for the biggest signing of all.
Last year, Sergio Ramos and Raphael Varane were let go to reduce the wage bill while in 2020, Real went without a single summer signing for the first time in 40 years.
More recently, they stepped back from the possibility of signing Erling Haaland from Borussia Dortmund, so convinced were they that Mbappe would come. They briefly thought they could buy both. Instead, they have ended up with neither.
In the short-term, Madrid will cope. They have won La Liga this season at a canter without Mbappe and they will win the Champions League without him too if they defeat a fatigued Liverpool in Paris next Saturday.
This season they have beaten Manchester City, Chelsea, PSG, Barcelona and Atletico Madrid.
And few would say Real's weakness is their attack. Vinicius Junior has emerged as one of the world's most frightening wingers and Karim Benzema established himself as the clear favourite for the Ballon d'Or.
With Mbappe in the team, would Vinicius and Benzema have shone so brightly?
But the longer term is where Real Madrid will surely feel the loss. Mbappe could have been a catalyst for change, a focal point for a new era, the kind of signing that forces a squad to update and tactics to be modernised.
Madrid's aura and history in the Champions League always makes them an opponent to be respected, but with Mbappe they would once again have been a team to be feared.
- Blow for La Liga -
President Florentino Perez will want to put things right quickly, to restore pride and prestige. Mohamed Salah, Harry Kane, Robert Lewandowski or a sensational return for Ronaldo could all quickly move to the top of his agenda.
If he wants to sting PSG, perhaps even Ousmane Dembele, reportedly destined for Paris from Barcelona, would appeal.
A more strategic approach might be to bolster other positions, like in midfield and at full-back, where Real actually need strengthening more.
For the current players, there could well be a galvanising effect.
The concern had been that an Mbappe announcement before the Champions League final might distract but a number of pointed, although well-disguised, social media posts on Saturday suggest Madrid's players would enjoy lifting a 14th European Cup now in Paris even more.
It would be a statement for Spanish football but La Liga, whose own relations with Madrid are hardly warm, have been hit hard by the decisions of Mbappe and Haaland to play, not just elsewhere, but for PSG and Manchester City.
"On Kylian Mbappe's possible announcement to stay at PSG, La Liga wishes to state that this type of agreement attacks the economic stability of European football," the league wrote in a statement on Saturday.
Amid the rise of state-backed clubs and the soaring wealth of the Premier League, Mbappe might have been an important moment of resistance, a check on the unfettered ability of those clubs to financially bulldoze the market.
For a long time, it was Real Madrid who had that power, an institution with the money and status to poach the world's best players, even from the world's top clubs, and there is certainly an irony in the anger now about another club throwing their weight, and money, around.
But their hurt is grounded in the wider point about what Mbappe's decision, coming so soon after Haaland's, really means.
Politically and financially, Real Madrid and Barcelona were for so long the top dogs in European football. Now it is the state-run clubs that rule the roost.
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