Romeo Lavia and Adam Armstrong condemned Chelsea to defeat as Southampton leapfrogged the lacklustre Blues in the Premier League.
Teenage midfielder Lavia became the first player born in 2004 to score in the Premier League with a fine strike at St Mary’s Stadium.
This was before Armstrong broke a 22-game duck as Saints stunned Chelsea.
The visitors had led through Raheem Sterling’s third goal in two games, only for Southampton to turn the game on its head in a challenging night for Thomas Tuchel’s side.
Chelsea might be a team in transition, with more big-name and big-money arrivals just around the corner courtesy of bullish new owners Todd Boehly and Behdad Eghbali.
But the Blues should still not be in the business of losing a clash like this against a remodelled and youthful Southampton.
Ralph Hasenhuttl’s rejuvenated Saints jumped ahead of Chelsea into seventh place in the table after the victory.
The Blues were left to lick the wounds of two costly defeats on their travels, just five games into the new campaign.
Chelsea can expect to slip further down the embryonic table after Wednesday’s fixtures, with Tuchel’s rebuilding job clearly boasting great promise but not yet delivering.
A central striker could hardly be more craved by a Chelsea side so creatively capable, but so far too reliant on Sterling’s finishing powers.
The Blues will now hope even more to complete the signing of Pierre-Emerick Aubameyang from Barcelona, while centre-back Wesley Fofana’s move from Leicester is expected to be announced shortly.
Chelsea settled faster and set about stretching the Saints set-up.
Sterling latched onto a neat through-ball, smartly shifting direction to create space and a fine opening.
But the former Manchester City star could only scuff his effort, gifting Ireland keeper Gavin Bazunu an easy save.
Sterling got round the back shortly afterwards in another sharp Chelsea move, but Mason Mount could not beat the defence to the cutback.
England forward Sterling continued to threaten by racing through, but the 27-year-old opted to go it alone rather than square to the unmarked Kai Havertz.
Sterling did finally put the Blues ahead with a calm finish on the turn after another incisive move from the visitors.
But just when Chelsea should have built on that 1-0 lead, Saints started to open up Tuchel’s team.
Lavia blasted home from the edge of the area after Cesar Azpilicueta failed to clear a corner with any conviction, with Edouard Mendy only able to palm the 18-year-old’s strike into the net.
Chelsea slipped off the pace after Saints’ equaliser, with the Londoners’ poise suddenly conspicuous by its absence.
Armstrong swept home to put Saints 2-1 up after Chelsea ceded possession cheaply in their own half, stunning the nonplussed Blues.
The Saints striker’s cool first half added-time finish allowed the hosts to turn around not just in the lead, but also in control.
Jorginho was so incensed by an unpunished challenge he received in the closing stages of the first half that he confronted referee Michael Oliver after the half-time whistle.
Azpilicueta had to lead the Italy midfielder away, with Chelsea in disarray at the break.
Mateo Kovacic replaced Ruben Loftus-Cheek at half-time, with Chelsea desperate to tighten up off the ball.
Southampton remained the more buoyant side however, with Mohamed Elyounoussi’s threatening header well blocked by Marc Cucurella.
Tuchel switched Chelsea to 3-5-2 and made a triple change with 20 minutes to play, with Ben Chilwell, Armando Broja and Christian Pulisic all joining the fray.
Pulisic stepped in at right wing-back, with Broja through the middle alongside Sterling.
Broja immediately showed the merits of a natural striker by overpowering the Saints defence, but another promising Chelsea attack came to nought.
Thiago Silva’s flicked back heel off the line denied Southampton a third goal, as the hosts sustained their overall second-half dominance.
Chelsea rallied with time running out though, and Broja forced a solid save from Bazunu when meeting Sterling’s cross.
The Blues pressed right to the death, with even Mendy joining the attack – but to no avail. Southampton were worthy winners in the event, with the Blues clearly cowed by the defeat.