Keane and Richards were in attendance at the Emirates to conduct their regular punditry duties with Sky Sports, and a video circulating on social media appeared to show the latter grabbing a man by the cuff of his shirt.
An eyewitness told The Express that the supporter had tried to headbutt Keane as the pundits made their way through Arsenal's Highbury Bar - which holds platinum members - towards the end of the game, and there was blood on the floor after the incident.
"The incident occurred shortly before the 90th-minute mark in the Highbury Bar for platinum members," the source told the publication. "The fan seemingly tried to head-butt Roy Keane.
"I then saw a mark between his eyebrows. Micah Richards stepped in and pushed the fan back against the door. I heard the altercation and I was shocked to see the individuals involved.
"Richards was very forceful at keeping the fan from attacking Keane and neither their security nor the door security could split the two apart. Keane was asking nearby staff to call the police.
"Additional security was called in and Richards and Keane followed until the fan was escorted downstairs via the escalator out of the building. Presumably, Keane and Richards then returned to their Sky Sports duties via the exit that they came in through next to where the video is shown.
"Afterwards, we noticed a small amount of blood on the floor on the route where the fan was led out. Seems like Micah Richards was acting as Roy Keane's bodyguard."
The Met Police confirmed that they were investigating the incident on Monday morning, and the force have now issued an update, stating that a man is being held in custody on suspicion of the alleged assault.
"Police are investigating an incident at Emirates Stadium on Sunday, 3 September, during which a man was assaulted," a statement read.
"On Monday, 4 September, a 42-year-old man was arrested on suspicion of assault (ABH). The man has been taken into police custody. Enquiries are ongoing."
Sky Sports also released their own statement on the matter, in which they revealed that Richards was trying to de-escalate the situation.
"We understand the police are investigating an alleged assault by a member of the public immediately preceding the footage circulating on social media. In the footage seen, Micah Richards was acting to defuse a situation," Sky's statement read.
Before the confrontation, Keane and Richards witnessed an all-action contest between the former's old club and Arsenal, who came from a goal behind to sink the Red Devils' 3-1 in spectacular fashion.
Marcus Rashford drew first blood at the Emirates, but Martin Odegaard immediately levelled before injury-time strikes from Declan Rice and Gabriel Jesus propelled Mikel Arteta's side to their third win from four games in the 2023-24 season.