The teenager's representatives wanted Gabriel to feature against Sabah in the Champions League, Brighton in the Carabao Cup and Bournemouth in the Premier League, according to the Manchester Evening News.
Gabriel's camp subsequently submitted a request for him to leave Manchester United, and the 15-year-old has not returned to Carrington since.
The demand came as tensions between the two sides increased over the youngster's pathway into the first team.
Gabriel was not included in the Champions League squad submitted by United to UEFA for the meeting with Sabah, meaning he was not eligible to make his senior debut in that fixture.
His camp had wanted him to be used for at least 20 minutes, while the Brighton match was also viewed as an opportunity for him to make history.
Manchester United had planned to give Gabriel his debut against Brighton, but his representatives submitted the request to leave before that game.
Had he played, Gabriel would have become the youngest player in Manchester United's history.
The teenager had already made his senior debut during pre-season when Michael Carrick brought him on against Atletico Madrid in Stockholm. He did not feature in another first-team friendly during the remainder of the summer.
Gabriel turns 16 on Tuesday and could have signed a scholarship agreement with United, but his camp has decided that his future lies elsewhere amid concerns over what they perceive as broken promises over playing opportunities.
United have continued to check on Gabriel's welfare since the request was made.
"We've got to be really careful of the whole situation," Carrick said last month.
"He's a young boy, he's 15, and for me, definitely, and from a club point of view, is looking after our younger players, their welfare, their well-being, their exposure, and I think we've got to be really, all of us, I think we've got to be really conscious of that, really, in this situation."
Gabriel has spent the last three weeks training in Barcelona with an individual coach, while Real Madrid and Barcelona are among the clubs interested in signing him.
United are considering legal action if he leaves in circumstances that breach regulations governing young players.
The club still holds Gabriel's registration and has made attempts to repair the relationship, but a departure is now expected.