The actual match hasn't started yet and it's one of the Premier League matches whereas Pre-season we're just thinking of it as a friendly and introducing Mount in his first match.
Yeah every player always needs to adapt to their new team, it's a process where they can adjust to the new coach's style of play but if Mount is developing well at Man United and fits Erik Ten Hag's tactics then the adaptation won't take long so he will play in the main squad.
Manchester united is a top club and they the best results inother to lift trophies. Manchester United defeated Leeds United yesterday, winning 2 nil thanks to overwhelming dominance. Erik Ten Hag have improved the performance of the club since his arrival, he is simply preparing the young and new players to become acquainted with one another, as they will pair as teammates on the field in the future. Mason Mount's performance was extraordinary but given that it was his first game as a Manchester United player, he will need more time for adjustment.