Arsenal slip to their first defeat of the season in second Champions League group game at French side Lens; Gabriel Jesus gives Gunners a first-half lead but Adrien Thomasson and Sepe Wahi turn the game on its head as Bukayo Saka goes off injured.
Arsenal were beaten for the first time this season as Lens completed a comeback win in the French side's first Champions League home game in 20 years.
The Gunners, who were unbeaten in their previous 11 games, took the lead in northern France when Gabriel Jesus steered home a fine early finish - but they were pegged back by Adrien Thomasson's equaliser after David Raya's poor pass.
Arsenal were then given a major injury scare when Bukayo Saka limped off just five days before a mouthwatering Premier League clash with Manchester City this Sunday, live on Sky Sports.
The Gunners looked off-colour and were made to pay when Sepe Wahi steered home the winner for Lens with just over 20 minutes to go.
Kai Havertz, Leandro Trossard, Reiss Nelson and Emile Smith Rowe came close for Arsenal, provoking manager Mikel Arteta to say the game was lost in both boxes.
"It's true we didn't generate enough threat in the final third," Arteta told TNT Sport. "Like in many occasions, we didn't provoke enough spaces. And just like in the PSV game, it was about the boxes. And we didn't defend the box well enough to win the game.
"You don't generate ten clear cut chances in the Champions League. When you get there, you have to put the ball in the back of the net. We have to defend the box much better than we have done."
After this result, the Gunners drop to second in the group, one point ahead of Sevilla after the Spanish side's draw at PSV Eindhoven.
Arsenal dillied and dallied
Arsenal were late arriving to Lens on Monday night with their flight to northern France delayed by around five hours, and it took the Gunners a little while to get going in the match, too.
Team news
Arsenal made two changes from the team who beat Bournemouth 4-0 on Saturday. Takehiro Tomiyasu replaced Ben White at right-back, while Leandro Trossard came in for Eddie Nketiah in attack.
The hosts had the early chances with Kevin Danso pulling an effort wide, before Thomasson's off-target header had Raya scrambling across his line.
But Arsenal benefited from a Lens mistake as Saka intercepted a loose pass to immediately free Jesus for a chance. The Brazilian striker took a touch out of his feet before rifling a shot into the far corner.
That goal silenced the home crowd and the Gunners nearly pounced for a second. Oleksandr Zinchenko freed Havertz in the area but his first-time volley was kept out by goalkeeper Brice Samba.
Thomasson missed another opening for Lens but made no mistake moments later - thanks to some sloppy play from the visitors.
Raya's clearance was intercepted by Deiver Machado, and the ball eventually fell for Thomasson who curled an expert finish past the Arsenal goalkeeper.
Then came a moment Arsenal did not want, Saka hobbling off holding his thigh with Fabio Vieira on in his place. The Gunners' momentum towards a second goal was dented, with a half-chance for Jesus representing their only opportunity from then until the break.
Player ratings
Arsenal:Raya (5); Tomiyasu (5); Saliba (6), Gabriel (6), Zinchenko (6); Rice (6), Odegaard (5), Havertz (5); Saka (4), Jesus (7), Trossard (6)
Subs:Vieira (5), Nelson (6), Smith Rowe (6), White (6) Nketiah (n/a)
Player of the match:Adrien Thomasson (Lens)
Arsenal came out of the blocks flying in the second half and nearly got an instant second with Trossard played in down the left, only for Samba to save the winger's low shot and subsequent ricochet off a defender.
But the Saka-less Arsenal failed to click into gear and Lens gave them a warning before their killer strike when Salis Abdul Samed curled into Raya's side netting after good work from Thomasson on the right.
Takehiro Tomiyasu saw a low shot saved by Samba from Martin Odegaard's corner and that stop proved crucial as Lens got their winner two minutes later.
Wing-back Przemyslaw Frankowsk set up striker Wahi on the edge of the area and he swept the French side into the lead to send the Stade Bollaert-Delelis into delirium.
Mikel Arteta then took off Odegaard, Havertz and Zinchenko - perhaps with one eye on Sunday's match with Premier League champions City - but the substitutes almost found an equaliser.
First, Smith Rowe cracked a 25-yard effort straight at Samba's palms, before Nelson saw what looked like a certain goal blocked on the line by Lens defender Jonathan Gradit.
Arteta: We need to learn from this
Arsenal manager Mikel Arteta:
"It certainly was [a tough game]. Congratulations to Lens. As we all saw, they are a really good side, it was an expectational atmosphere and a very difficult opponent.
"We did the most difficult thing, scoring the first goal, had total control of the game and then in one action they managed to score and it was game on again.
"Mistakes happen, it's part of football. They will continue to be there somehow. We had a huge chance straight after the final third with Leo. We had two or three big chances.
"[We need to] learn from it and it was a big night for us. We didn't manage to get the result that we wanted. We will take a lot of things from it and learn."
What's next?
Arsenal v Manchester City
Sunday 8th October 4:00pm Kick off 4:30pm
Arsenal's next assignment is the small matter of a Premier League clash with current champions Manchester City at Emirates Stadium, which will take place live on Sky Sports Main Event on Sunday October 8 (kick-off 4.30pm).
Up next for Lens is a Ligue 1 meeting with LOSC Lille, also on Sunday (kick-off 4.05pm).
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