Gabriel is unable to make his United debut this season due to Premier League rules. He was eligible to feature in the Carabao Cup and FA Cup this term, but United's early elimination from both competitions means he will have to wait until 2026-27 to make his senior bow.
While interim boss, Fletcher revealed why Gabriel wasn't selected for their FA Cup third round clash with Brighton. He said: "JJ is a fantastic talent. I've enjoyed working with him immensely, getting to know him. He's a young lad with a massive sort of hype, you know there's a lot of noise around him and deservedly so because he's a real talented kid.
"He works hard. First thing I'd say is he loves football. He loves training, he loves playing, he loves having the ball, he loves expressing himself. He makes fantastic decisions. He's got an enthusiasm for the game that's just amazing. 15-year-old, a bright future ahead of him. I'm super excited by his talent, but the most important thing is that he keeps developing, and he's in the U18s team, and he's doing fantastically well, but he's still got lots to learn.
"He's developing within there. But the world's his oyster and he just needs to go on his journey and his time will come in due course. But for just now, it's a little bit early (to put him in the first team) I think and I think a lot of people would agree with me.
"But I couldn't speak highly enough of JJ. He's got an amazing family and amazing support around him and he's somebody we're delighted to have at the club.
"And we just hope he keeps improving, learning every day and just keep being the boy that he is really and enjoying love and playing football and going and expressing themselves and scoring lots of goals so the Manchester United U18s win every game. And then I'll be a happy man."