Southampton seized on defensive frailties to win 3-1 at Stamford Bridge, leaving Chelsea 10 points behind fledgling leader Manchester City and only four points above the relegation zone.
The result made Mourinho himself raise the prospect of Chelsea firing him — just as owner Roman Abramovich did in 2007 after three years of the Portuguese in charge. However, he said he would not walk away from the challenge of turning things around.
“If the club wants to sack me, they have to sack me because I’m not running away,” he said. “If the club sacks me they sack the best manager that this club has had.”
Mourinho was brought back in 2013 and he delivered the title by eight points in May.
But the team has rarely looked like champions this season, not even at Stamford Bridge. Two of Chelsea’s losses have come at home, where Mourinho had only been beaten once in the league before this season across his two spells as manager.
Chelsea was booed off as Southampton won at Stamford Bridge for the first time in almost 14 years.
Captain John Terry was restored but Saints exposed the now familiar failings in the defense, which just five months ago formed the foundation for winning the title.
Chelsea has conceded at least two goals in seven of their eight Premier League games this season and time and again Sadio Mane, Graziano Pelle and Dusan Tadic found holes in the Chelsea defense.
Willian scored a fourth free-kick goal in four games to open the scoring, but Chelsea afterward scarcely tested goalkeeper Maarten Stekelenburg.
Southampton equalized after Ramires was booked for a foul on Mane, and Jose Fonte played the ball long to Pelle, with his back to goal.
Pelle held off defender Gary Cahill, chested it and, one bounce later, Davis hit it sweetly to score.
Mourinho was incensed when Radamel Falcao was booked for diving in the area after a collision with Stekelenburg, and Southampton went ahead shortly afterward when Cahill lost the ball and Terry failed to intercept Pelle’s pass to Mane, who sneaked in behind to slot into the net on the hour.
Mane ran at Chelsea and then played in Pelle for the third goal in the 72nd.
While Southampton also had a clear penalty appeal in the first half when Branislav Ivanovic pulled down Virgil van Dijk by his shirt, Mourinho blasted referee Robert Madley over the Falcao incident.
“The penalty is a giant penalty, and he was afraid to give (it),” Mourinho said. “The referees are afraid to give decisions for Chelsea. Why? Because when they give, there is always a question mark from you (the media). … We are always punished.”
Mourinho was also questioned about his decision to take off Nemanja Matic just 28 minutes after he brought him on as a halftime substitute, but denied he had tried to humiliate the player.
“I don’t do that to anyone in football and in life,” Mourinho said.
Chelsea’s crisis overshadowed an impressive performance from Southampton.
“The whole team performance was perfect,” manager Ronald Koeman said. “After 1-0 down we looked comfortable and of course to win against teams like Chelsea you need your best players on that high level.
“Sadio was dangerous in his movements. It was impossible to have good defending against him.”