The 16-year-old joined the Red Devils in a £9.3million deal from Monaco last week, and is set to sign a professional contract when he turns 17 in January.
United beat off competition from Arsenal, Tottenham and Leicester to land the highly-rated youngster.
Mejbri uploaded a picture on his Instagram page with his name on the back of a No.7 United top, which suggests it could be the number he wears for the club’s Under-18s during the current campaign.
The midfielder will be hopeful of first-team opportunities under Ole Gunnar Solskjaer in the future.
Solskjaer has put his faith in youth since replacing Jose Mourinho in the Old Trafford hotseat in December.
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The Norwegian brought in Daniel James and Aaron Wan-Bissaka, both 21, for a combined total of £68million earlier this summer.
And he has confirmed 17-year-old Mason Greenwood will be involved in his plans this term following Romelu Lukaku’s departure to Inter Milan.
Discussing his trust in United’s younger players, Solskjaer said: “I believe in this way of managing this football club. I am paid to manage – and this is not just about me getting results now to help my reputation or whatever.
“It is about the club going forward in the right direction.
“If we don’t make these decisions now, they will have to be done in 12 months’ time or 18 months or 24 months – because at some point these kids will have to be a part of Man United.