There will be few sleepless nights at Arsenal over this result and, as their fans were keen to point out, first place in the Premier League is a bigger attraction than Carabao Cup progress. But Mikel Arteta will still be frustrated by defeat to an alert, bold Brighton team who responded superbly to going behind and have a bright future if a young side’s performance is any measure. Kaoru Mitoma completed the turnaround after Danny Welbeck’s penalty had cancelled out an Eddie Nketiah finish and Tariq Lamptey added some gloss. It meant Arsenal’s 12-match winning run at the Emirates, Brighton coincidentally the previous opponent to win here, came to an end. The relatively strong sides Arteta picked for Europa League action had raised comment in previous weeks but he took few risks with key players here. Only William Saliba remained from the team that won at Stamford Bridge on Sunday and, while the bench was packed with big hitters, Bukayo Saka was given a rare night with his feet up. The news would have been music to Gareth Southgate’s ears. Roberto De Zerbi had made eight changes from the rollercoaster victory over Wolves and, as so often on these occasions, it appeared the winner would be whoever could muster the best semblance of cohesion. Nketiah’s goal made that proposition look flawed, because Brighton had carried the early threat. The young Paraguayan Julio Enciso had failed to connect with a free header in front of goal after three minutes and blasted just over before the opener. Jeremy Sarmiento had cut inside and fizzed a shot of his own a shade high and, beyond a skied effort from Fábio Vieira from Reiss Nelson’s cutback, Arsenal had not got going. Nelson has an appreciable spring in his step after scoring twice against Nottingham Forest last month. It was a contribution that sparked a flagging career and his work in teeing up Nketiah this time was outstanding. Carrying the ball inside, he picked the right timing and weight of pass to play in Nketiah to his left; the finish, swept first time around Jason Steele, was sublime. But Brighton deserved their equaliser, which arrived seven minutes later, and it was a moment for Karl Hein to forget. The 20-year-old already has 16 caps for Estonia and a burgeoning reputation but, on his first-team debut for Arsenal, slipped as Welbeck ran on to a through pass. The former Arsenal forward appeared to have taken an overly heavy touch but Hein’s error allowed him to reach the ball first, the keeper clipping him after he did so. The penalty was dispatched calmly. The opening half-hour’s rattling tempo could not be sustained, although Arsenal may have gone back in front before the interval if a breaking Nketiah had successfully picked out Vieira. They would have led four minutes into the second period had Nketiah’s low 20-yarder, which thudded off the post, been a few inches to the right. Arsenal’s intensity was unrecognisable and, almost immediately, Nelson demanded an exceptional save of Steele with a header from Marquinhos’s cross. Albert Sambi Lokonga then flashed wide and at this point Brighton, who had introduced Mitoma and Pascal Gross at the interval, were only just holding on. In a role reversal of the first half, De Zerbi’s side promptly scored with their first attack since changing ends. The clinical right-foot finish by Mitoma was similar to Welbeck’s, too, with a strong run and pass from the impressive Jeremy Sarmiento creating the chance after a smart counter. Arteta promptly looked to the bench and called upon Gabriel Martinelli. Before the Brazilian could have any impact, Lamptey settled things. It was another slick goal, taking a Billy Gilmour pass in his stride before sprinting clear down the left and slotting past Hein. More Arsenal cavalry duly arrived but Brighton’s progress was merited.