Chelsea coach Guus Hiddink insists his side cannot afford to make sloppy mistakes against Barcelona on Wednesday or they will be "exposed and punished".
Hiddink's side strolled to a comfortable 3-1 victory over west London neighbours Fulham with goals from Nicolas Anelka, Florent Malouda and Didier Drogba.
"You cannot afford to make the slightest error," said Hiddink. "I was not happy because we were sloppy in some positions and that's what I didn't like despite a good win."
He added: "It's not just one position, it was a sequence of errors mostly. If we want to play a little bit for ourselves when we are not fully concentrated, then we give away chances, like we did against Fulham, but, if we do that on a higher level, you will be exposed and punished.
"I was happy with the result but I don't shut my eyes to more moments in the game where we were sloppy."
Hiddink replaced defender Alex and midfielder Michael Essien at half-time but insisted both of those will be fit for the Champions League semi-final return leg at Stamford Bridge on Wednesday - the first match in Catalonia having finished 0-0.
"Alex had a slight problem with his groin but I think he will be okay," Hiddink continued. "We also wanted to protect Michael and that is why we made the change.
"We wanted to take the game in the league between our matches against Barcelona very seriously but hopefully we can be in two finals at the end of May - that would be perfect."
Hiddink was pleased to see Anelka and Drogba produce some fine link-up play but refused to say whether the pair would both be involved against the Catalans on Wednesday.
"It gives us more options," said Hiddink. "If you are a central defender, you wouldn't want to face them, they are strong and fast but we didn't just choose this game, we play them together in practice as well."
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