Match ends, Chelsea 1, Southampton 3.
Chelsea's terrible start to the season continued as Southampton came from behind to record a stunning victory at Stamford Bridge.
The Premier League champions took an early lead through Willian's free-kick.
Steven Davis drove in an equaliser before Sadio Mane ran through after a defensive error to put Saints ahead.
Graziano Pelle's angled finish sealed victory for Southampton, who climb to ninth in the table. Chelsea's fourth defeat of the season leaves them 16th.
The Blues have taken only eight points from a possible 24 this season and are only four above the relegation zone.
Chelsea's defensive frailties exposed again
Chelsea's struggles this season have been exacerbated by an uncharacteristically poor defence. They have now conceded 17 league goals - two more than they did in the whole of Mourinho's first season with the club in 2004-05.
Following Chelsea's defeat by Porto in the Champions League on Tuesday, fans called for the return of captain John Terry, and they got their wish on Saturday as the 34-year-old was restored to the centre of defence alongside Gary Cahill.
However, Terry's presence did not solve Chelsea's problems. Branislav Ivanovic and Ramires were fortunate to avoid giving away penalties with clumsy challenges, while Davis was allowed to run unchallenged to score the equaliser from Pelle's chested pass.
After the break, Cahill gave the ball away to Dusan Tadic and Terry failed to cut out his pass, enabling Mane to put Saints in front.
Mane was a constant menace to the Blues defence, which backed off to allow the forward to set up Pelle for Southampton's third.
With Diego Costa serving the last game of a three-match suspension, the visit of Southampton provided Radamel Falcao with a chance to make a case for a regular starting place.
He has scored only one goal since joining on loan from Monaco in July and, based on his showing against Southampton, it is unlikely he will be threatening Costa any time soon.
Although Falcao had a strong claim for a penalty when he fell under a challenge by Saints goalkeeper Maarten Stekelenburg, he did not trouble the visitors enough and managed a solitary shot in 90 minutes.
Big decisions going against Chelsea?
Mourinho was incensed that no penalty was given for the incident involving Falcao, claiming that officials are "afraid" to give decisions for Chelsea.
He said: "It was a big penalty for us. Not small, not doubtful. Big. Huge.
"That was a penalty. We don't get decisions and I think referees are afraid to give decisions to us.
"When we are top I understand everyone wants to push you down, but when you are down give us a break."
Can Saints build on last season?
Southampton performed beyond expectations to finish seventh in the Premier League last season, but they had won only two of their first seven games this term before the trip to Chelsea.
However, the manner of this victory - their first at Stamford Bridge since 2002 - will give them confidence of challenging for a top-seven finish again.
They rarely looked troubled by the Blues, with Mane and Pelle combining to devastating effect in attack. One or both of them played a part in all three goals.
Southampton are also a tough side to break down - only Everton have beaten them by more than one goal in the Premier League this season.
Man of the match: Sadio Mane
What they said
Southampton manager Ronald Koeman: "If we can show this against Chelsea we can show it much more.
"We had a difficult start to the game but we came back and we made it 1-1 with a great goal. The difference was how we came out of the dressing room, pressed Chelsea and had fantastic movement.
"We totally deserve three points today."
The stats you need to know
- Chelsea have conceded at least two goals in seven of their eight Premier League games this season (the other game was a 2-0 win against Arsenal)
- Chelsea have conceded as many goals in their last four home league games (seven) as they had in the previous 17 at Stamford Bridge
- Chelsea have eight points from eight games. The average finishing position from this start is 14th
- This is Chelsea's lowest points total after eight games in a top-flight season since 1978-79 (four points), when a win earned two points. Chelsea were relegated that season
- Chelsea did not have a shot in the second half until the 77th minute
Chelsea entertain Aston Villa in the Premier League on 17 October, when Southampton host Leicester.
- 7Santos do NascimentoBooked at 43minsSubstituted forMaticat 45'minutesSubstituted forRemyat 73'minutes
- 22WillianSubstituted forPedroat 64'minutes
- 10E Hazard
- 9FalcaoBooked at 57mins
- 2Cédric Soares
- 6José Fonte
- 17van Dijk
- 21BertrandBooked at 26mins
- 14RomeuBooked at 9minsSubstituted forWard-Prowseat 45'minutesBooked at 77mins
- 10ManéBooked at 34minsSubstituted forYoshidaat 90+2'minutes
- 11TadicSubstituted forRodriguezat 78'minutes
- 19PellèBooked at 86mins
- Robert Madley
- Shots on Target
Second Half ends, Chelsea 1, Southampton 3.
Offside, Chelsea. John Terry tries a through ball, but Oscar is caught offside.
Substitution, Southampton. Maya Yoshida replaces Sadio Mané.
Oscar (Chelsea) wins a free kick in the defensive half.
Foul by Sadio Mané (Southampton).
Corner, Chelsea. Conceded by Ryan Bertrand.
Attempt blocked. Falcao (Chelsea) right footed shot from outside the box is blocked.
Graziano Pellè (Southampton) is shown the yellow card for a bad foul.
Eden Hazard (Chelsea) wins a free kick in the attacking half.
Foul by Graziano Pellè (Southampton).
Corner, Chelsea. Conceded by Jose Fonte.
Attempt missed. Pedro (Chelsea) right footed shot from the left side of the box is close, but misses to the right. Assisted by Loïc Remy.
Offside, Southampton. Sadio Mané tries a through ball, but Jay Rodriguez is caught offside.
Attempt missed. Cesc Fàbregas (Chelsea) right footed shot from outside the box is close, but misses to the right. Assisted by Pedro.
Substitution, Southampton. Jay Rodriguez replaces Dusan Tadic.
James Ward-Prowse (Southampton) is shown the yellow card.
Attempt missed. Oscar (Chelsea) right footed shot from the right side of the box misses to the left. Assisted by Eden Hazard.
Corner, Southampton. Conceded by John Terry.
Attempt blocked. Dusan Tadic (Southampton) left footed shot from the centre of the box is blocked. Assisted by Sadio Mané.