An Arsenal fan was arrested after red wine was thrown towards the Manchester United dugout during Saturday's Premier League clash.
Police said a 49-year-old man was held after the incident during Arsenal's 2-1 defeat to United at the Emirates.
United substitutes were involved in angry exchanges with supporters behind them after the wine was thrown as Louis Van Gaal's side celebrated their opening goal.
The wine is believed to have splashed some of the players on the United bench.
Eight other people - all believed to have been United fans - were also arrested during the pulsating game on Saturday evening.
A Met police spokesman said: "Nine people were arrested at the game. Six were arrested for entering the playing area, two for missile throwing and one for racially aggravated harassment.
"A 49-year-old man was arrested on suspicion of missile throwing at the Manchester United bench. He has been bailed until a date in December."
Arsenal confirmed a man was arrested for the incident and said he will be banned from attending games if he is found guilty.
United's former manager Sir Alex Ferguson previously complained that the bench at the Emirates does not offer enough protection from hostile fans.
He said in 2008: "It is too open for my liking. It should not happen when you build a modern stadium.
"You need tin helmets. It is a weakness in the planning that you don’t provide safety for the visiting substitutes and staff.”
It is unclear whether United will lodge an official complaint over the incident.
