According to BBC Sport, Gabriel did not train with Michael Carrick's squad at the end of last week amid a breakdown in the relationship between his family and United, and had been expected to make his competitive debut in Wednesday's Carabao Cup tie against Brighton before the situation escalated.
The precise cause of the rift has not been made public, with sources at United saying they are not sure what has led to it developing as it has.
Gabriel has sent formal notice that he wants to cancel his registration with immediate effect, a request United retain the right to challenge. Chief executive Omar Berrada and director of football Jason Wilcox are directly involved in talks with the player's family, underlining how seriously the club regard the situation, and Manchester United have not given up hope of resolving matters given Gabriel remains under contract until the end of the season.
Born in London, Gabriel was only 14 when this season began but has already made a rapid rise through United's age groups. He won the Premier League Under-18 Player of the Year award last term as well as United's own Under-18 accolade, having scored twice on his debut for the Under-18 side at just 14 and playing an integral role as United reached both the FA Youth Cup and Premier League Under-18 finals.
He made his senior bow in a pre-season friendly against Atlético Madrid in Stockholm last month, then scored a hat-trick on his Uefa Youth League debut just last week. Having progressed into United's Premier League 2 squad this season, he scored sensational goals in each of his first two matches at that level.
Gabriel does not turn 16 until October 6, the date from which he would become eligible to sign a scholarship agreement with the club. United chiefs Wilcox and Berrada had previously moved to secure his long-term future during a tense meeting last year, fending off interest from elsewhere, making this fresh breakdown all the more unexpected given how far the relationship appeared to have progressed since.
Nicknamed "Kid Messi" by those who have watched his rapid progress through United's academy, Gabriel has attracted comparisons to some of the finest talents to come through the club's youth system in recent years. His social media following and a sponsorship deal with Nike reflect the level of hype already surrounding a player who has yet to turn 16, adding an extra layer of scrutiny to a situation United will be desperate to resolve quickly and quietly.