At 15, the winger is too young to play in the Premier League - but can turn out in the FA Cup.
But Fletcher has poured cold water on the prospect of a debut today: "JJ is a great talent and I have enjoyed working with him.
"There's a lot of talk about him and it's fully deserved, because he's a really talented kid who works hard and loves football.
"He makes amazing decisions and has an enthusiasm for the game, which is absolutely incredible. I am very excited about his talent and he has a bright future ahead of him.
"But the most important thing is that he continues his development, because he still has a lot to learn.
"His time will probably come, but right now, in my opinion, it's a little too soon, and I think many will agree with me.
"However, I can't praise JJ enough. He has an amazing family and great support around him and he is a player we are very happy to have in the club.
"The world is at his feet and he just has to continue his journey."