When Atlético Madrid scored against Real Madrid in the city derby at the Vicente Calderón in January 2015 and the old place erupted, a tiny ballboy in a white bib came running along the touchline from his position near the bench and leapt into the arms of the coach, Diego Simeone. On Saturday night, 10 years on, the pair of them will be as close but if Atlético score at the Santiago Bernabéu don’t expect the same embrace. Not even if the kid scores it, and he might. If he does, it would mean everything: “Atlético,” he says, “is the team of my life.”

His name is Giuliano and he was 12 then. He is 22 now and these days he plays for Atlético, still flying up the wing, looking like his lungs might explode. He has become the embodiment of Atlético’s identity during a season in which they are fighting for everything. He is also an academy product and Simeone’s son, however much the manager tries to avoid that fact.

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Giuliano joined Atlético at 16, having spent his early teenage years with River Plate in Argentina, moving into the family home in La Finca, west of the city, but father and son no longer live together since he returned from a loan spell at Alavés. He and his father sit apart when they go to see Gianluca Simeone, Diego’s son and Giuliano’s brother, play for fourth-tier Rayo Majadahonda. And it has been fun to watch Simeone Sr not celebrate when his son scores and go wild when anyone else does, not least because it is happening increasingly often.

But if Simeone once said he could never sign a son of his and is concerned about being accused of favouritism, aware of the risk of dressing room divisions, he need not worry any more. Not just because Giuliano is an academy product rather than a signing, as discreet as his dad, as fiercely competitive as him too, but because when he starts Saturday night’s derby on the right side of midfield it will be for all the right reasons. “I don’t see a son; I see a footballer,” Atlético’s coach insists, and so does everyone else. A very good one too.

A ballboy almost a decade ago, Simeone is making his first derby appearance as a player. Like Kylian Mbappé, he sat it out when these two great rivals teams met at the Metropolitano in the autumn, a late Ángel Correa goal securing a draw. Now, both will start at the Bernabéu, any doubts there might have been before gone now. After a difficult start, Mbappé has scored 21 times in 33 games. He has nine in his past nine league matches, and two in his past three in the Champions League. Simeone has scored three and set up two more in his past three, decisive in all three competitions.

If Mbappé’s recovery was always likely, if Real Madrid coming into this derby top of the table was just logic imposed again, the same was not true of Simeone. This is a decisive derby: Real Madrid and Atlético are separated by a single point, while Barcelona are only three behind that. That owes much to Barcelona’s collapse, a title that seemed as if it was in their hands after October’s clásico slipping away with three consecutive home defeats, and Mbappé becoming Mbappé again. It also owes much to the ballboy who started this season as a sub, the decision to loan him out again only reversed late in the summer, and is now irreplaceable.

Atlético were Spain’s big spenders in the summer, Julián Álvarez and Alex Sørloth arriving for more than €100m between them. They were joined by Robin Le Normand and Conor Gallagher. That brought excitement but it needed time. It also meant few opportunities for Simeone Jr. In the opening 11 league games of the season, he played 0, 0, 0, 6, 14, 5, 65, 0, 0, 33 and 20 minutes, starting only once. He didn’t start either of their first three games in the Champions League, two of which they lost. At the end of week 11, Atlético were 10 points behind Barcelona, four behind Real Madrid and trailed Villarreal, having won only five times.

The solution, it turned out, was at home. Simeone started the following week and the revival began. It has not all been about Giuliano of course. Rodrigo de Paul looks like the Argentina player, not the man who had the fans whistling him. Álvarez has been outstanding, scorer of 16 goals. Antoine Griezmann has 15. And Sørloth is on 11, mostly from the bench, his contribution key.

More importantly, the coach has found a structure; he has also found a collective idea, a commitment, getting his players to truly buy into his idea that they will all play a part – something he says “isn’t difficult; it’s almost impossible”. Atlético have strength and variety in depth, the team that most use their subs and that get the most from them: a third of their goals have come from the bench.

Simeone Jr symbolises that, something in his energy, his intensity, which transforms the entire team, in his dad’s words. Beaten only once in the 19 games he has started, they went on a run of 15 successive wins in all competitions. The run ended at Leganés, knocking them back off the top of the table, and a draw with Villarreal followed, but he was then decisive against Salzburg, Mallorca and Getafe, taking Atlético into the top eight of the Champions League, the semi-final of the Copa del Rey and to within a point of Madrid at the top of the table.

Asked how they arrive at this derby this week, his dad replied, “by bus”, and then started laughing like Muttley, very pleased with his joke. The real answer is: in very good shape.

In a week marked by arguments over referees, Madrid meanwhile come into it with injuries: Mbappé will be there, despite a bruised calf, Jude Bellingham will too, and Carlo Ancelotti hopes that Eduardo Camavinga will be available in time. They are though without Dani Carvajal, Éder Militão, David Alaba and Antonio Rüdiger; there will be at least one midfielder in the back four, probably two. There is also, the coach concedes, the old question about balance, a defence exposed; but then there’s hat forward line. “I could take off one, two, three, four attackers and press more,” Ancelotti said. “But if I do that, I don’t score 60 goals. It’s simple.”

“They have great individuals,” Simeone insisted –four times. “We have to take the game to a place where we can hurt them.”

And so it is set, Ancelotti against Simeone again. Against two of them this time, just like 2015. He comes with back-up, fathers and sons entering the fray: Davide Ancelotti, Carlo’s assistant coach, will be there too. This is the 26th meeting between two coaches so classic they bring their kids with them now. No current managers have met as often nor been so well matched: they have won nine each, and it’s a battle every time, they know.

“These are the games you like being at,” Madrid’s manager said. “There’s high pressure, yes, but everyone is going to watch it, it’s a great spectacle, there are great players on the pitch, it will be very competitive. It’s spectacular and you’re in the middle of it. As a coach you’re in the most uncomfortable place in the ground but you suffer, you fight with your team. You have to enjoy it before. And hopefully celebrate it after.”