In France they're saying it's - just about - a done deal. The St Etienne defender will sign for Arsenal in the coming days. If not hours. Two Sundays ago, Saliba's minders struck terms over a five-year contract with the Gunners. The Londoners followed that up this week with an offer to ASSE of €25m down plus a further €5m in bonuses. With the board rating Saliba in the €30m class, haggling is now in it's final stretch.
And to see Saliba's potential confirmed by this Premier League move will be another personal triumph for Mbappe Snr. A native of Bondy, as a junior Saliba turned out for his hometown club. The same AS Bondy where Mbappe brought through both sons Kylian and William just two years earlier.
"He was my coach for one or two years," recalls Saliba. "He was a real coach! He taught me everything and if I'm here today, it's thanks to him."
While there was a three-year difference, Saliba and the Mbappe brothers did become close, "Since his father was my coach, I often went to his (Kylian's) home.
"Before he went to a private college, we were in the same elementary school. He already showed that he was above the others..."
Indeed, while Mbappe would be scouted and recruited to the Clarefontaine academy, Saliba was largely ignored by Ligue 1 clubs. Right up to the age of 15, trials with PSG, Bordeaux, Le Havre and Auxerre left him with nothing and he would continue in local football at FC Montfermeil - where a positional switch to centre-half changed everything.
Fabio Frasconi, who coached Saliba for two seasons at AS Bondy, says: "That's where (at Montfermeil) he exploded for two seasons.
"Once settled in central defence, he became monstrous. Even when he was pressed by three players, he was never afraid. And if it went bad for his team, he went into attack where he'd create a huge mess, scoring or causing a penalty."
With a background in futsal, Saliba's ease on the ball and eagerness to attack - even when starting at centre-half - brought him to the attention of top-flight scouts. ASSE eventually winning his signature ahead of Troyes.
Three years on, which included Saliba landing a new deal to 2023 last year, and that 'explosive' progress he showed at Montfermeil has continued. And in no small part due to those advisers around him.
"It's impressive that we'll speak for more than half an hour about the tactical aspect of his position," says Frasconi, who is always available to advise his former protege. "You sometimes get the feeling of interacting with a guy with ten years of professional career behind him."
A reflection mirrored by Saliba's senior teammates. Now with two seasons of first team football under his belt, Saliba is firmly a first-choice.
"William has a very good state of mind. He is listening, he is trying to progress. He takes in the information, it's positive. He has many qualities."
At 18 and standing at 1.93m with his ability on the ball, the attraction for Arsenal is obvious. But it's that temperament, as Ruffier refers to, that has convinced the Gunners to push through a deal now - rather than play a game of wait-and-see.
Bernard Diomede, Saliba's France U20 coach, has no doubts the teen has shown he can handle the step up.
"Jean-Louis (Vannuchi, France U18 coach) told me that he had made him play in different positions, especially on the right side. That's why he's here today with us. Today he shows some cold blood. He offers a certain serenity when you see him play for a young person of his age. That's what Jean-Louis confirmed to me. He likes to say that he is a cold player, who does not panic despite the opponents he faces, despite the levels where he can evolve.
"These are criteria that are important when you play games for France. When you play L1 in front of 30,000, 35,000 or 40,000 supporters. It's important to know how to handle the pressure. All the feedback I have on him has come together."
Which you fancy is the same "feedback" Arsenal are working from. Even at 18, for those in charge of transfer policy in North London, just as Diomede says, Saliba has the mentality to handle this next stage in his career.
And watching on will be those coaches from Bondy, including Frasconi and Mbappe. After Kylian's success, the Parisian minnows may've produced another world-beater for French football.