The 16-year-old had moved to Manchester from Spanish side Gazte Berriak, near Pamplona, at the age of 13, and was tipped for a bright future having received call-ups for both England's U16s and Spain's youth set-up over the past year.
Last season, Osuchukwu was on the bench a number of times for Manchester United's U18s and was part of the squad that won the U16 Premier Shield, while also being invited to train alongside the first-team squad on occasion, a rare honour for a player of his age. Barcelona and Reims had both held interest in the goalkeeper, but with his family keen to return to the Basque region, Athletic Club emerged as the favoured destination for the switch.
Announcing his departure on Instagram, Osuchukwu wrote an emotional message reflecting on his time at Old Trafford.
"After four unforgettable years at this incredible club, I have made the incredibly difficult decision to move on and embrace a new challenge. This was not a decision my family and I took lightly. While I was grateful to be given the opportunity to continue my journey here, we believe the time is right to begin a new chapter," he wrote.
He continued by thanking those who had supported his development at the club.
"It is impossible to put into words how much this club has meant to me. I want to thank everyone at Manchester United—and I truly mean everyone. From the coaches who believed in me, to the kitmen who worked tirelessly behind the scenes, and especially my team-mates, who became so much more than teammates—they became my brothers.
"Every one of you has played a part in shaping not only the footballer I am today, but also the person I have become. The lessons, values, friendships, and memories I have gained here will stay with me for the rest of my life."
Osuchukwu also thanked the club directly for the opportunity to represent it.
"I also want to thank the club for believing in me and giving me the opportunity to wear this badge. Representing Manchester United has been one of the greatest privileges of my young career, and something I will always be proud of. Although I am moving on, Manchester United will always hold a special place in my heart. I leave with nothing but gratitude, respect, and memories that I will cherish forever. Thank you for everything," he wrote.