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.
Spurs failed to have a shot on target in the first half but improved in the second period, with Richarlison twice seeing efforts ruled out for offside.
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.
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
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.
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.
Against the run of play entirely, Real Sociedad had their equaliser when a cross from Idriozola found Kevin Rodrigues free at the back post and he drove a volley at Keylor Navas.
He won a penalty after Lindleof took him out of it in the penalty area, which Casemiro buried it. It ended 1-1 in a game that was hard to gauge due to the number of changes on either side.
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.
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.
The visitors made another fast start at the beginning of the second half, with Doucoure quickest to react when Mykolenko's volley came back off a post.
The keeper managed to palm away from his own goalline, but Aubameyang was lurking to pull the loose ball back to Smith Rowe in front of goal, and his low shot squirmed in off thepost.
The home side added a second on 53 minutes when Hwang Hee-chan picked out an easy finish for Matheus Cunha at the far post and the Brazilian then turned provider when Craig Dawson poked home a low cross.
The visitors failed to clear a corner and the ball fell for Haaland at the back post to smash home a fierce first-time strike that Pickford got a hand to, but could not keep out.
However, Cagliari got back into the game when a Jakub Jankto corner found Alberto Dossena to steer the header in at the far post. This ended a run of six consecutive Serie A clean sheets for the Bianconeri, 615 minutes of football.