Raheem Sterling was once again on the scoresheet as the Blues made a positive start, but the vulnerabilities seen in that heavy defeat against Leeds at Elland Road were once again on show from Thomas Tuchel’s side.
Romeo Lavia produced a wonderful finish from the edge of the box to equalise before Adam Armstrong sent the Saints into the break ahead as he fired in from close range.
There was no real second-half response from Chelsea, which will strengthen the calls for investment in forward areas before the transfer window shuts on Thursday night.
Chelsea made a decent start at St Mary’s, defying a dreadful playing surface to put together a number of nice moves. Sterling was played in by Mason Mount and fired straight at the goalkeeper, before Hakim Ziyech scuffed his effort when free in the box.
Another chance came and went for Sterling but it was third time lucky, as he put the Blues in front midway through the first-half. Mount’s cut-back found his England team-mate in the box and he took his time before firing low into the bottom corner.
That lead lasted just five minutes though, before a corner was cleared out to Lavia on the edge of the box and he bent a lovely effort past Edouard Mendy for his first Southampton goal.
And the Saints turned it around completely as they went into the break in front. Romain Perraud pulled the ball back to Armstrong and the striker’s finish was good enough to beat Mendy in first-half stoppage-time.
Southampton went close to doubling their advantage after the break, as Mohamed Elyounoussi looked certain to head home at the back post but Marc Cucurella was there to produce a sensational block.
Kai Havertz headed over a decent chance with his final touch of the match as Armando Broja was introduced on his return to St Mary’s, before Thiago Silva produced a great clearance on the line to deny Mohammed Salisu.
Broja was bright and had a header saved but Southampton saw out the remainder of the match with little in the way of real scares, leaving the Blues with even more questions to ponder as they head into a hectic run of fixtures.