Gabriel, who turned 16 on October 6, reportedly asked Manchester United to cancel his academy registration in September.
The teenager's desire to leave came just before the Carabao Cup tie against Brighton, where he was expected to make his competitive debut under Carrick, who was ready to put faith in him.
However, after the news broke, Carrick refused to hand Gabriel the chance, and his side went on to lose the game and be knocked out of the cup.
The problem for United now comes as Gabriel has not signed a professional contract, so the club cannot simply set a normal transfer price.
Carrick could receive only around £350,000 in training compensation if Gabriel moves to a European club, rather than a conventional transfer fee, which would be a crushing blow.
The Guardian revealed that an offer of a formal scholarship for next season has now been lodged with the player, but this does not mean he will stay with the side.
Any Premier League club must now negotiate a fee directly with United for the player, and if he is sold, United could then expect to receive in excess of £3m, plus a clause in the deal granting them 20% of his next transfer fee, according to the report.