That loss comes just two weeks after a 1-0 defeat at Aston Villa and adds to the domestic reverses at the hands of Everton, Newcastle United and Stoke City in which Mourinho believes his players were below their best because of the stature of the contest.
"The easiest thing to do is to play well, is to perform, when you are not playing for something big," Mourinho said.
"The difficult thing is to perform when you feel the heat. Because of that some players are big and some are not so big.
"I don't think it's courage. I think it's personality qualities.
"You have players for everything and you have other players that are in their habitat in some circumstances."
Chelsea take on French giants Paris Saint-Germain in the Champions League quarter-final first leg on Wednesday before returning to domestic action with a home fixture against Stoke on Saturday.
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