Teenager Marcus Rashford scored twice and set up the third as Manchester United hurt Arsenal's Premier League title challenge at Old Trafford.
Rashford, who scored twice on his debut against FC Midtjylland on Thursday, guided in a loose ball for the opener then headed in three minutes later.
Danny Welbeck nodded Arsenal back into the game, but Rashford picked out Ander Herrera whose shot was deflected in.
Mesut Ozil's volley gave third-placed Arsenal hope, but United held on.
Arsenal - who have not won the title since 2004 - remain in third place, five points behind leaders Leicester having now played the same number of matches.
The Red Devils moved back into fifth position with their first win in three league games.
Manchester United manager Louis van Gaal fell over in front of the fourth official in what was described as "the comedy moment of the season" by BBC commentator Conor McNamara.
The Dutchman, who rarely leaves his seat in the dugout during matches, appeared to be annoyed by an possible dive by Gunners striker Alexis Sanchez.
"I have had a lot of criticism because I am sitting on my bench and now I am off my bench and I have lot of criticism because I'm doing that," Van Gaal said.