According to the information, the talented winger will not be in David Gordo's 23 man squad and will be in Luis de la Fuente's for the doubleheader against Germany and Montenegro on the 10th and 15th October. The first one is a friendly and the second is a qualification match for the Euros.
Therefore, Ernesto Valverde wouldn't lose his new star for three weeks (the World Cup is in Brazil) and, also, he would be fresh for the first 'Clásico' of the season, which will be played on 26th October.
Barca, who are doing everything to stop him from playing for the u17s, would get their reward. Ansu Fati would also experience something very special by making his debut for the u21s and playing for a team higher than his age group.
Only one person would lose out and that is David Gordo, who was excited about the prospect of having the Barcelona player at the World Cup after finding out his nationality.