Kelleher also had a lucky escape when he punched Ake's cross on to Foden, the rebound crashing against the bar, while Jeremy Doku struck thepost late on.
The Korean scored five minutes later by tucking in a finish after the Hammers failed to clear a loose ball, but then hit thepost with a similar chance as West Ham turned over possession straight after the resulting kick-off.
Romelu Lukaku put the Red Devils in front shortly after the break but Choupo-Moting levelled in the 63rd minute, heading in Xherdan Shaqiri's corner at the back post.
Brighton continued to press, with James Milner hitting thepost and Facundo Buonanotte having a goal disallowed for offside, before Richarlison also clipped the woodwork for the visitors.
Arsenal could muster little in the way of a response as they limply succumbed to a defeat more chastening than the one they dealt out to Antonio Conte's men in September, when the prospect of Wenger besting an elite opponent in the dugout briefly
Steven Berghuis reminded Brighton not to let their guard down as he struck thepost for Ajax, but that was one of few chances for the visitors as the Seagulls closed out a comfortable win.
Natalya sent Naomi into thepost, then rolled her back in the ring and locked in the Sharpshooter, forcing Naomi to tap out. - Dolph Ziggler came out and once again insulted the fans for only liking gimmicks and elaborate entrances.
Luis Enrique's side dominated and thought they had doubled their lead in the 48th minute, but Ousmane Dembele's fine curling finish was ruled out by the video assistant referee for a foul in the build up.
It was a chance for the likes of Pereira and Anthony Martial to prove themselves and Martial did just that when he beat three Real Madrid players to set Jesse Lingard up at the back post at the end of the first half to give United the lead.
The 'Yellow Submarine', managed by ex-Gunners Unai Emery, will go into the second leg on top, but facing the prospect that just one unanswered concession will send them out.
Justin doubled his personal tally in spectacular fashion shortly after the hour mark, meeting Ndidi's left-wing delivery with a swerving volley that beat David Raya via the inside of thepost.
They should have added more before the interval, but Pablo Fornals hit thepost and Sebastien Haller slipped at the crucial moment after rounding goalkeeper Dean Henderson - who was making his first Premier League start for the visitors.
Kai Havertz nicked the ball off the Spaniards toes to race away on goal, but he blazed over when one-on-one. It was Chelsea's turn to hand outthe gifts next though, and it led to Smith Rowe scoring the only goal of the game.