At 18 years and 125 days old, the winger became the youngest player to start a match between the two sides in the Premier League.
Saka emerged in the Gunners first-team last season, making his debut in the Europa League against Vorskla and then playing the full match against Qarabag.
He was instrumental in the 3-0 victory over Eintracht Frankfurt in the Europa League earlier this month, scoring once and setting up another two goals.
Saka started his first Premier League match last week when played the first 45 minutes of their 3-2 win over Aston Villa.
Arsenal headed into the game against United in good form, having won their last three matches, while their opponents approached the clash off the back of a 2-0 defeat to West Ham and a 1-1 draw with Rochdale in the Carabao Cup.
Their underwhelming start to the campaign saw Ole Gunnar Solskjaer's team sit 11th in the table before Monday's encounter, and it was the first time in almost 30 years they have been in the bottom half of the top-flight going into a game against the Gunners.
1989 – Tonight is the first top-flight meeting between Man Utd and Arsenal with the Red Devils starting the day in the bottom half of the table since 3rd December 1989, when Arsenal won 1-0 at Highbury. Decline. #MUNARS pic.twitter.com/IHGXwQ4WAA— OptaJoe (@OptaJoe) September 30, 2019
But United had the history of the fixture on their side, as the Red Devils have not lost to Arsenal at home since 2006 - winning eight and drawing four of the last 12 games at Old Trafford.
Meanwhile, the Red Devils saw midfielder Paul Pogba make his 100th Premier League appearance, having been kept out of the meetings with Leicester and West Ham due to an ankle injury.
Pogba contributed to 49 goals in his first 99 appearances, netting 24 of his own and setting up a further 25.