Southampton did not look like the team with a caretaker in charge. Liberated after terminating their brief, tortured relationship with Nathan Jones, the Premier League’s bottom side turned up at Stamford Bridge, found Graham Potter in a muddle and emerged triumphant after punishing Chelsea’s complacency with a beautiful free-kick from James Ward-Prowse. It has been a while since Southampton, who are three points off 17th place, have shown this much spirit and organisation. The transformation was remarkable. Rubén Sellés had been given less than a week to put a plan in place, but Southampton, who had lost nine of their previous 10 league games, were inspired, fighting hard and somehow holding firm during a frantic second half. “With the players, I need to be honest with them and tell them what to expect,” Sellés said. “I demand the best. This team is alive. If anyone wants to beat us then they’ll have to be at their best.” Chelsea were nowhere near their top level. January’s big spenders, drifting in 10th place after winning two of their past 14 games, got what they deserved for picking a team that would have been more suitable for the early rounds of the Carabao Cup. There was no cohesion; only a performance to test the insistence that Chelsea are building under Potter. The crowd’s anger at full time felt ominous. The boos were loud, Potter copping much of the abuse. Several fans near the tunnel called for him to quit and despite all the calls for patience he is not making the most of his deep, expensive squad. “After a 1-0 defeat at home any criticism is understandable,” he said. “We’ve had a tough period and are integrating a lot of young players. A lot of people will say I’m the problem and I’m not saying that their opinion is not worth articulating. My job is to work.” Chelsea, who rested Thiago Silva, Reece James, Marc Cucurella, Ruben Loftus-Cheek and Hakim Ziyech, had the wrong attitude. Potter made six changes after losing to Borussia Dortmund and the result was a messy start, with the absence of Silva and James unsettling the defence. Southampton were hungrier, Sellés delivering a constant barrage of instructions from the touchline, and there was a hint of what lay in store in the third minute, a chance arriving when Kamaldeen Sulemana seized on Benoît Badiashile’s error and forced Kepa Arrizabalaga to intervene. Chelsea, who were relieved César Azpilicueta had regained consciousness after going off with a nasty head injury, were disorganised. Arrizabalaga saved from Sulemana and Kalidou Koulibaly cleared the rebound from Stuart Armstrong. Southampton clearly fancied their chances. Roméo Lavia and Ward-Prowse, who would cross for Paul Onuachu to head wide, offered dynamism in midfield. With Enzo Fernández and Mateo Kovacic overrun in midfield, Chelsea could not find João Félix. Nothing worked. Noni Madueke tested Gavin Bazunu, but was quiet on the right. Mason Mount struggled. Worn down by Southampton’s pressing, Chelsea were slow to the loose balls. Koulibaly could have been sent off for catching Onuachu and David Datro Fofana, a raw 20-year-old making his first start since joining from Molde last month, never looked capable of shaking off Jan Bednarek and Armel Bella-Kotchap. What must Pierre-Emerick Aubameyang make of being overlooked again? Chelsea plodded on without Aubameyang, creating nothing, asking for trouble. Southampton gained belief and, just as the board went up to show four added minutes, they won a free-kick when Azpilicueta fouled Armstrong. The ball was 25 yards out, slightly to the left, made for Ward-Prowse. Arrizabalaga knew what to expect. The goalkeeper was still left grasping thin air when Ward-Prowse whipped the free-kick into the bottom-right corner. Potter responded, Wesley Fofana and Raheem Sterling on for Koulibaly and Datro Fofana at half-time. Chelsea soon threw more millions off the bench, introducing Kai Havertz and Mykhaylo Mudryk for Mount and Madueke. Havertz combined with Sterling. An equaliser looked certain until Ainsley Maitland-Niles blocked Sterling’s low drive. Southampton defended brilliantly. With 20 minutes left Félix crossed for Sterling, whose header was hooked off the line by Romain Perraud. Chelsea had 12 minutes of added time after Sékou Mara, attempting a bicycle kick, almost took Azpilicueta’s head off. Conor Gallagher tested Bazunu, but Southampton remained defiant. They can suddenly spy a route to safety.