Chelsea crashed to a sensational 2-1 defeat to Crystal Palace at Stamford Bridge this afternoon, that calls into question not only the Blues's ability to retain their title, but if they can get into the Champions League places at all.
Four points from four games is not quite crisis point at the moment but it does suggest that getting into the top four might be a step too far this season.
An even first first half exploded into life in the 65th minute when Bakary Sako exposed a porous Chelsea rearguard.
Mourinho sent in forward's Radamel Falcao and Kenedy and it resulted in a brilliant Falcao equaliser only for Joel Ward to nod home into an empty net with nine minutes of the clock left.
The Blues have not won at home this season - an opening day draw with Swansea setting the scene for a fitful sequence of performances.
The last time Chelsea lost at home was also a 2-1 defeat to Sunderland in April 2014.
But four points from four points is not champions form and a crisis of confidence is growing within SW6.
An enterprising start by both sides, neither giving an inch with the individual tussles but also getting forward whenever possible.
Palace certainly not holding back and giving the ball cheaply to the Blues.
But the first effort on goal, came on 13 minutes from the boot of Cesc Fabregas. His long-range effort failed to trouble Alex McCarthy in goal.
Some sublime counter-attacking play by Chelsea on 20 minutes cut the Eagles apart. The ball fell to Pedro, on the edge of the penalty area, but the curling left-footer, inched just wide of goal.
The blues though they had a penalty on 16 minutes when some shirt-pulling by Conor Wickham on Kurt Zouma went unnoticed in the penalty area by referee Craig Pawson
Palace's first strike on goal, a counter-attack came from the left boot of Bakary Sako, on 26 minutes, but Thibaut Courtois successfully tipped the effort around the post for a corner.
A brilliant attack by Palace should have seen them take the lead on 30 minutes, when Wilfred Zaha opened up the Chelsea rearguard. The ball fell to in-running Yohan Cabaye, whose first-time shot, on target, was beaten away by Courtois.
Three minutes before the break, the Blues were thwarted by two excellent diving saves by McCarthey, the first, an angled blaster from Costa, the second, a follow-up by Fabregas.
McCarthy was then on the spot to snap up a weak, albeit, on target daisy-cutter from Nemanja Matic.
Chelsea had a claim for another penalty turned down on 54 minutes when Damien Delaney looked hopefully at the linesman after his sliding challenge on Costa appeared to have upended the striker. But the defender struck lucky as a corner was given instead.
At the other end Wickham should have tapped in a Sako byline cross but somehow mistimed his effort and hacked the ball instead into Courtois's midriff.
Sako then forced Courtois into a diving save. Zaha's day ended in 55 minutes as Yannick Bolasie replaced him.
Pedro was next up to shoot, but, after a mazy run by Costa, the Spain forward's shot was deflected for a corner. From the resultant kick, Gary Cahill saw a header cleared off the line by Sako
Within seconds, on 66minutes, Sako had put the visitors' ahead. A quick break on the left saw Pape Souare pass to Bolasie and his cross was met by a combination of Cesar Azpilicueta and Sako. The ball fell kindly for him and he lifted the ball over Courtois and into the net.
Jose Mourinho made a double change bringing in Radamel Falcao and new boy Kenedy.
This was quickly followed by Rubén Loftus-Cheek for Matic.
Bolasie missed an open goal with 18 minutes on the clock, as the Blues left themselves exposed at the back, in search of a game-saving goal.
And it worked and how. A brilliant pass by Fabregas to Pedro on the right saw an inch-perfect cross to the in-running Falcao, whose diving header zipped past McCarthy at the near post.
But, in a search for a winner, the Blues left themselves open at the back again and a clip back by Sako on 83 minutes fed Joel Ward who tucked home a neat header past Courtois to restore the visitors' lead.
Despite half chances and excellent last-gasp defending, the visitors took the points back to South-East London.
Teams: Chelsea, Courtois, Ivanovic, Cahill, Zouma, Azpilicueta, Matic, Fabregas, Pedro, Willian, Hazard, Costa,
Subs: Begovic, Baba, Mikel, Loftus-Cheek, Kenedy, Remy, Falcao
Palace: McCarthy, Ward, Dann, Delaney, Souare, McArthur, Cabaye, Zaha, Puncheon, Sako, Wickham,
subs: Hennessey, Mariappa, Chung-Yong Lee, Mutch, Ledley, Bolasie, Gayle
Referee Craig Pawson