After featuring heavily for both the U23s and U18s so far this season, the Madrid-born winger was named among the substitutes for the match with the Canaries.
Speaking to MUTV ahead of Saturday's Premier League clash with Norwich City, Rangnick explained the Argentine's inclusion.
He said: “[Garnacho] has been training with us [the first team] for the last two weeks and I know that he is one of the most promising players.
“[He is] still only 17, but a player that hopefully if he keeps his feet on the ground. If he keeps trying to get better every day, then he will not only be sitting on the bench but there is every chance he will be playing for this club.