Solskjaer said in his post-match press conference after Huddersfield game, talking about Europa League, "the kids will get chances, definitely, to play games, that's for sure". Then went on to say they would try to find something positive about the Europa League situation, which might be giving game time to Greenwood, Chong, Garner (the 3 names he mentioned).
So in answer to your question, I think we will see Garner and the other 3 chosen youngsters (Greenwood, Gomes, Chong) feature in Europa League and domestic Cup competitions, then if they perform well, or if we have an injury crisis, they might feature in the PL.
I think Garner could manage a few starts in the PL, if the matches are chosen carefully but there are some away games and some opponents that would be difficult for him at this stage, due to the physical battle in midfield. If you remember when Rashford first broke into the team under LVG, he had some brilliant games and some in which you'd barely notice him, as it was a teenage boy against men.
United have also briefed the media that they do not intend to buy players that would block the development path of their most talented youngsters; that could be viewed in several different ways. If Garner is currently 7th choice for midfield behind Matic, Herrera, Pogba, Fred, McTominay and Pereira, I would expect him to be either 7th or 6th at the start of next season, once all the departures and arrivals have taken place.
If you look at his first team involvement this season. He was on the bench for the CL matches at Valencia and PSG, also the FA Cup game against Reading and came on as a late sub against Palace when we were 3-1 up. In contrast, Solskjaer has been far more willing to risk Chong and Greenwood.