Waiting to demonstrate his qualities in Spain, the teenager, with one senior cap for Argentina, has already made an impression in his native land.

“Franco’s passage through our club left an indelible mark.” Roberto Binzuna, the president of Cemento Armado, where Mastantuono earned his stripes before joining River, tells BBC Sport.

“Only time will decide how long it lives on in our memory.

“He’s a sensational young man, with outstanding human and sporting characteristics, distinctive in whatever sport he played – an exceptional tennis player and an even better footballer.

“His presence always stands out over the other players, even the older ones, and he has an incredible shot on him.”

Indeed, Mastantuono’s ability to strike the ball from range has caught the eye. Most notable was his free-kick against Superclasico rivals Boca Juniors in April, swept into the top corner from about 30 yards.

That aside, the left-footer is agile, quickly shifting the ball one way and the other when dribbling. Such has been evident from the start.

“I remember he was restless behind the ball. But what I saw set him apart,” says Marcelo Olariaga, the vice president of Club Atletico River Plate Azuleno, Mastantuono’s first home.

“Compared to the rest, he ran very lightly with the ball. But he always had it tied to his feet.”

Although the teenager often starts on the right wing, much of his impact comes in central positions.

In River’s first Club World Cup outing this summer, a 3-1 win against Urawa Red Diamonds, Mastantuono roamed inside and bent a pass to the left, eventually leading to Facundo Colidio’s opening goal.

As for the statistics, Mastantuono has featured 63 times for River, scoring 10 goals and assisting seven. At 16, he was the youngest to score for the Argentine giants with the famous red sash.

Meanwhile, he is the youngest to feature competitively for Argentina’s senior team, reaching that landmark against Chile earlier this month.