Raheem Sterling put Chelsea in front in the 23rd minute with his third goal in two games but Southampton leveled within five minutes with a spectacular strike from 18-year-old Romeo Lavia. In first-half stoppage time the home side took a lead they would not relinquish courtesy of a deflected effort from Adam Armstrong.
The result means Chelsea drop to eighth and below Southampton in the Premier League table after suffering their second defeat in five games to start the season.
Yet Chelsea began the game brightly and could have taken an even earlier lead. Sterling failed to make the most of two openings in the Southampton box, first dispatching a tame shot at goal and then being denied by a saving tackle after opting to go alone rather than making the most of his side’s numerical advantage on the break.
At the third time of asking, though, the summer signing from Manchester City made no mistake. After Southampton were left exposed by a smart Chelsea move down the left, Mason Mount slid the ball across the box and, despite a momentary intervention by Romain Perraud, Sterling was able to turn and find the back of the net.
Despite Chelsea’s early superiority, their lead would be short-lived. Left free on the edge of the box following a half-cleared corner, Lavia scored his first senior goal in some style with an unstoppable drive past Edouard Mendy. The goal marked the first time a player born in 2004 had scored in the Premier League.
Just before half-time the hosts had the lead. Chelsea’s defending left something to be desired as Perraud drilled a low ball into the box where Armstrong was left unmarked as his left-foot shot took a slight deflection off Chelsea defender Kalidou Koulibaly to help it past Mendy and into the top corner of the net.
It was Southampton that twice came close to extending their lead in the second half. Two last-gasp defensive efforts by Chelsea, from Marc Cucurella and Thiago Silva, were required to keep the deficit to just a single goal.
After starting with four at the back, Chelsea boss Thomas Tuchel made a triple substitution and a move to a back five midway through the second half, bringing on Christian Pulisic — as a right wingback– as well as Ben Chilwell and Armando Broja.
But the changes did little to improve a disjointed Chelsea performance as they went down to a defeat that means they are already playing catch up to the teams at the top of the table.