The incident happened soon after Pierre Emerick Aubameyang had converted a 69th-minute punishment to allow the Gunners a 2-0 lead against United.
After hammering Smalling, the guy hurried together with his arms towards the Gunners players observing their goal.
Smalling was unhurt and also the fan was shot off the pitch and also arrested.
An Arsenal spokesperson said:”We completely condemn the behavior of the individual that conducted on to the pitch and approached Chris Smalling throughout the match.
“We’d like to apologise to Chris and Manchester United, also are pleased that [that the spectator] was arrested.
“The individual will also be banned from Arsenal matches home and off. Our matchday staff have ongoing training on tackling episodes like this.”
Right following the match, an announcement by the Metropolitan Police said:”a person was detained on suspicion of common attack and pitch invasion following an episode throughout the Arsenal v Man Utd game at the Emirates Stadium today.
Arsenal manager Unai Emery reported the episode was something he”did not need” to watch and predicted all buffs to show”respect”.
The episode came just hours after Aston Villa midfielder Jack Grealish was hit in the face by a spectator throughout Villa’s 1-0 derby triumph over Birmingham City at St Andrew’s.