Manager Ruben Amorim called Mather up due to concerns about Noussair Mazraoui's fitness.
Mazraoui recently returned from injury and has played in the last two Premier League games for Manchester United, against Manchester City and Chelsea.
However, he has not been participating in training this week, which has raised questions about his availability for the Brentford game.
Mather was previously set to join Kayserispor, but the move was blocked by Manchester United.
Additionally, he received an offer from an unnamed club in the UAE Pro League, but now he might be involved in the upcoming match.
Mather was also absent from Manchester United's U21 match against Athletic Bilbao on Wednesday night.