The move continues the club's pursuit of talent from outside the traditional Basque catchment area, according to Noticias de Navarra. The teenager qualifies under Athletic's cantera philosophy through his formative years at Gazte Berriak in Ansoáin, rather than his place of birth.
Osuchukwu, whose parents are Nigerian, moved with his family to Navarre as a young child, where he began his goalkeeping education at Gazte Berriak in Ansoáin and also trained with Osasuna's youth ranks. It was that spell in Navarre, rather than his birthplace, that made him eligible for Athletic Club under the club's long-standing recruitment criteria. His family later relocated to England for work, a move that led to him joining Manchester United's academy, where he spent four years developing under the Old Trafford setup before his departure this summer.
Athletic had not confirmed the deal, but Osuchukwu himself broke the news on social media, posting photos from the signing alongside his first comments as an Athletic player: "I feel deeply honoured and proud to begin this new stage with Athletic Club, a club with a rich tradition and values.
"I want to thank my family for their love and support throughout this journey. None of this would have been possible without them. This is only the beginning. Aupa Athletic."
The post drew a warm response from fellow young players including Elijah Gift and Christian Imga, as well as Enzo Alves, son of Real Madrid legend Marcelo.
Osuchukwu has already represented Spain's Under-17 side, featuring at the Algarve Tournament, which the side won, and in qualifying for the Under-17 European Championship. Manchester United had reportedly hoped to keep him, but the player and those around him judged Athletic's goalkeeping pathway the better route towards senior football, beating interest from several other European clubs.
Before completing his exit from Manchester United, Osuchukwu also posted a farewell message on social media thanking the club, describing the chance to represent the Red Devils as one of the greatest privileges of his young career and expressing gratitude to the coaches, staff and teammates who worked with him during his four years in the academy.
According to Marca, the move is seen at San Mamés as part of the club's long-term succession planning behind Unai Simón, with the teenager regarded internally as one of the more promising goalkeeping prospects of his generation.