It was deal that was only announced after the market had shut. And one that no-one, bar those in charge of Chelsea and St Etienne, had seen coming. Amougou arriving at Cobham in THE late hours of Monday afternoon for a medical and to sign forms.
For their part, St Etienne didn't kick up a fuss. For a player, at 19, inside the final 18 months of his contract, Chelsea's offer of €15m cash fell nicely in the range of what ASSE were seeking. With time ticking in the early hours of deadline day Monday morning, Chelsea's proposal was accepted and Amougou was cleared to fly to London to complete the move.
As we say, ASSE management saw it as a deal to be made. Before Christmas, the promoted L1 club had offered Amougou a new contract to 2029 at triple his €15,000-a-month wages. But Amougou and his management team could sense momentum was with him, interest from abroad was growing and so the teen declined.
That decision arrived amid enquiries from German champions Bayer Leverkusen. The interest being garnered from that momentum Amougou had built up over the first-half of his debut season as a senior player.
With just the one L2 appearance to his name from last season, Amougou had established himself as a key part of former ASSE coach Olivier Dall'Oglio's plans this term. Strong in the tackle and with a clean distribution, Dall'Oglio had no hesitation fielding the teen at the base of his midfield as ASSE sought to re-establish themselves as a top-flight club.
And with that ability comes a striking maturity, which further convinced Dall-Oglio that Amougou could handle the demands of Ligue 1 football.
"We know the quality of Mathis in the midfield," stated Dall-Oglio before his dismissal in December. "Last season, he had a very busy season, between coming to join the professionals at 17, going to play international matches, going to play the U17 World Cup, passing his school certificate, being asked to go a little bit to the right and left. It does a lot of things for a young player.
"So last season, he was training with us regularly. But then I felt a boy released, actually. Released already from school, freed from a lot of constraints and therefore more available to play with the group on the field."
And as mentioned, tipping the balance in Dall'Oglio's thinking was the player's maturity.
He continued: "And I felt that he had more maturity now in his game.
"I think he went up a notch. He is on the axis of progression, I think that for now he can give the last pass, finishing, he is a boy who will be able at some point to score goals, even if he is a working midfielder, a defensive mid.
"He is very good at the pass, but I think that in the areas of truth where it tightens, I think that he can still work on things and he will be able to progress further on his long game."
Dall-Oglio's words surely were reflected in the notebooks of Paul Winstanly and Lawrence Stewart, Chelsea's co-directors. Amouogou is one for the future, certainly. But he also has the maturity and approach which makes the midfielder one Enzo Maresca, the Blues manager, can rely upon if needed.
In France, amongst most scribes, they expect to see Amougou back in L1 next season with Chelsea's French partners Strasbourg. The coming six months seen by many back home as little more than a extended trial and acclimatisation period.
But not so for the youngster's former teammates. Louis Mouton, who shared a midfield with Amougou this season, is convinced the France U17 World Cup finalist has everything in his armoury to make it at Chelsea.
"Concerning Mathis, he joins a big club," stated Mouton this week. "I don't have any doubt that it will go very well for him. I wish him the best."
For Dall-Oglio's successor, the Norwegian Eirik Horneland, Amougou leaves with his and the club's best wishes. As mentioned, both Horneland and the board knew they were on borrowed time with Amougou and the timing of Chelsea's offer was welcome.
"Am I disappointed to see Mathis Amougou leave? You can see it in two ways," stated Horneland. "We can note that we managed to develop a rather interesting player who has tapped into the eye of Chelsea, which does not hesitate to put a lot of money on promising young players.
"They had several meetings with Saint-Etienne. Today, Chelsea has paid a fairly interesting amount to recruit him. Our strategy is of course to develop young players but there will also come a time when we will have to sell them. Mathis Amougou was really interested in joining Chelsea and today we can simply wish him good luck."
A deal to suit all parties? It is looking that way. But as we say, it's all come at Mathis Amougou in a rush. That famed maturity will surely be needed if he is to carry on the momentum created over these past six months.