Our first midweek match of the season served up an evening to forget when a Chelsea lead was overturned to consign us to a second defeat in our five games played.
The first half was going well when Raheem Sterling scored his third goal in two games and other opportunities were being created, with the returning Hakim Ziyech at the centre of a lot of the good play at that stage, but by half-time it was the home side in the lead as the Blues defending from a set-piece and then in open play was exposed for goals by Romeo Lavia and Adam Armstrong.
Thomas Tuchel made substitutions in the second half, one enforced by injury, but the Blues lacked the cohesion to threaten enough in a stadium where we won well last season.
This result gives Southampton their first league win against us at St Mary’s since 2013 and but for Marc Cucurella and Thiago Silva goal-line blocks the margin could have been more.
The Blues will be looking for a quick improvement ahead of a weekend London derby and the start of our Champions League campaign.
Reece James was ruled out due to illness and Trevoh Chalobah had an injury from his appearance at the weekend but Kalidou Koulibaly had served a suspension so came straight back in.
Skipper Cesar Azpilicueta came in for James and with Conor Gallagher the latest suspended player, there was a chance for Hakim Ziyech as Tuchel made three changes from the Leicester win.
Having suffered an emphatic defeat at Leeds in our last game away from Stamford Bridge, Chelsea were seeking a solid start to this one at the other end of the country.
Saints attacking midfielder Mohamed Elyounoussi tested Edou Mendy early on but our keeper was up to the challenge.
Raheem Sterling did not catch his shot properly when it opened up for Chelsea for the first time, with Saturday's two-goal man fed by Mason Mount and cutting inside his marker before his scuffed strike. Ziyech too did not make the most of a Mount pass soon after but Chelsea were starting to make an impact on a pitch surprisingly worn for this early stage of the season.
Sterling had begun the game in a central position in the front three but he was out on the right 20 minutes in when part of a silky move with Ziyech. It gave Mount his first chance to score on a ground where he enjoys doing so so much. The Saints defended that, as they did moments later when a good Armel Bella-Kotchap tackle stopped a goalbound Sterling, but they could not keep our summer signing out for much longer.
Havertz, in a left-side role tonight, created a chance for Mount to seek out Sterling in the middle. Although our no.17 was initially crowded out, he kept his cool and swivelled before finding the net from close range. 0-1.
Lead let slip
A fabulous Ziyech pass gave Havertz the chance for a rapid doubling of the lead immediately after but from an angle he hit his shot into the goalie’s hands. Instead the Chelsea advantage lasted just five minutes.
A preventable corner was conceded and when it was half-cleared, Lavia sent an unstoppable shot back past Mendy from the edge of the penalty area.
By half-time we were in arrears. The added minutes had just been announced when we lost possession and the Saints stretched our defence with a series of passes and when the ball came in to Adam Armstrong, he had been left in too much space. The shot took a deflection that helped it past Mendy.
Thomas Tuchel brought Mateo Kovacic on in place of Ruben Loftus-Cheek for the start of the second half. The Croatian, on his way back from injury, had been deemed not fit enough yet to start a game. Now it was Loftus-Cheek who appeared on the bench soon after the restart with an ice pack on his leg.
The Blues could have been a further goal in arrears but for Cucurella in the right place to block a close-range header from Elyounoussi. Mendy tipped over the same player’s follow-up effort.
Tuchel’s side did not make the most of one promising counter-attack and with little else to show as a response to the Southampton lead, our head coach made a triple substitution with Ben Chilwell, Christian Pulisic and, back on the ground where he played last season, Armando Broja all introduced.
Still it was Southampton coming closer to scoring again, with Thiago Silva somehow hooking a goalbound header away from underneath the crossbar.
Inside the last 10 minutes, a Sterling cross did allow Broja a far-post header but well marked, the Albanian international could not generate the power to beat Cavin Bazunu. Sterling also picked out the keeper after an exchange of passes with Mount and his clear frustration in doing so reflected much of the night.
Moved from its previously scheduled Sunday slot due to our Champions League programe, we take on West Ham at Stamford Bridge in a 3pm Saturday league encounter.
Chelsea (4-3-3): Mendy; Azpilicueta (c) (Broja 66), Thiago Silva, Koulibaly, Cucurella; Loftus-Cheek (Kovacic h-t), Jorginho (Chilwell 66), Mount; Ziyech, Sterling, Havertz (Pulisic 66).
Unused subs Kepa, Ampadu, Chukwuemeka, Gilmour, Vale.
Scorer Sterling 23
Booked Pulisic 80
Southampton (4-2-3-1):Bazunu; Walker-Peters, Bella-Kotchap, Salisu, Perraud (Djenepo 73); Lavia (Aribo 59), Diallo; Elyounoussi, Ward-Prowse (c), A Armstrong (Lyanco 86); Adams.
Unused subs McCarthy, Bednarek, Valery, S Armstrong, Walcott, Mara.
Scorers Lavia 28, A Armstrong 45+1
Booked Salisu 53, Ward-Prowse 64
Referee Michael Oliver