"We have nothing to argue with, we have no defence because our performance in the first half was so bad," Mourinho said after Chelsea lost 1-0 at Real Betis on Tuesday.
"Betis deserved the victory because of the way they played in the first half and the way they defended in the second half."
It was also Chelsea's first defeat in regular playing time for six months, although they were knocked out of the English League Cup last week after losing on penalties to Charlton Athletic after a 1-1 extra time draw.
The Primera Liga side, who were crushed 4-0 by Chelsea when the two sides met at Stamford Bridge two weeks ago, earned a deserved victory when substitute Dani scored three minutes after stepping on to the pitch to replace injured Brazilian striker Ricardo Oliveira.
The Spaniards had to weather a fierce storm from Chelsea in the second half, but they held firm and managed to inflict a first defeat on the English champions.
Mourinho admitted that he was not happy about the way the match had been refereed, insisting that Luxembourg match official Alain Hamer had allowed the Betis players to waste too much time during the second half.
But he added that his complaints were overshadowed by his concerns about his side's performance.
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GOOD POSITION
But the Portuguese manager said the defeat did little to affect his side's chances of progressing to the knockout stage of the Champions League.
"Liverpool and Betis play each other in the next game and one of them is going to concede points so when you look at the standings we are still in a good position."
Betis coach Llorenc Serra Ferrer said he was proud of his team's performance, but seriously worried by injuries to defender Nano and leading striker Oliveira who both had to leave the pitch early on in the first half.
"For Betis taking part in the Champions League is a reason for celebration and beating Chelsea, the most powerful team in the world, is very pleasing," he said.
"I'm very happy with the way the team played and with the support of the fans, but concerned that the injuries have left us in a critical situation.
"The injuries are too high a price to pay for a victory like this."
A spokesman for Betis said that it was feared that Oliveira, who was the club's leading scorer last season, had sustained serious knee ligament damage after clashing with Chelsea defender Ricardo Carvalho midway through the first half.