The youngster joins the list of 17-year-olds to have played in the Premier League for the Reds, by coming on as a substitute.
He replaced Manuel Ugarte in the 75th minute of a frustrating encounter at Old Trafford, and operated on the right wing.
Mantato, who is very versatile, scored the winning goal against Arsenal in the FA Youth Cup semi-final at the Emirates Stadium back in February.
The England Under-17 international can be extremely proud of his achievement, even if the 1-1 result was a disappointing one.
Angel Gomes is the club's youngest player to appear in the Premier League, while Chido Obi holds the record in the division for the Reds as a starter, when picked in the XI against Tottenham Hotspur last February.