Both sides raced to the incident to support their team-mates – but it only ended up causing more trouble.
That’s after a missile, which appeared to be a coin, was thrown in the direction of the nearby United players.
Jesse Lingard reacted furiously to the incident, while Ashley Young picked up the object and handed it to the referee.
United were leading 3-1 at the time of the incident, following goals from Alexis Sanchez, Lingard and Anthony Martial.
Pierre-Emerick Aubamayeng had given Arsenal hope of a comeback but substitute Martial’s goal ended the hosts’ challenge.
The win makes it eight-in-a-row for Ole Gunnar Solskjaer.
His record stands at six Premier League and two FA Cup successes in his temporary stint in the top job.
BBC pundit Phil Neville said on the 3-1 win: “It was a brilliant performance from United.
“I think what we have seen from Ole is that it’s more than just giving people love it is about tactical awareness.
“The two substitutions at the end there sealed the game, brilliant performance from United.”
Ian Wright said: “Manchester United look like different players.
“I think what we have seen from Ole is that it’s more than just giving people love it is about tactical awareness”
“The way they are playing, they couldn’t have been enjoying it under Jose Mourinho at the end.
“We’re seeing tremendous performances.
“Arsenal were trying to score a perfect goal but they just needed to get the ball into the box more. I was thinking at some stage were Arsenal going to drop back a bit but it never happened.
“They did deserve it tonight United, it was a poor performance from Arsenal.”