Sahin, who has just returned to the club from Liverpool, was part of the side humbled 4-1 at home by Hamburg at the weekend.
“It was a very painful defeat because we should not be letting in four goals at home,” Sahin told reporters.
“We can make it all good again on Wednesday but we have to be very focused because Shakhtar are a team that will punish us if we make mistakes.
“There has to be a marked improvement in our performance.”
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Chelsea have struggled to build momentum in the opening stages of the campaign and are currently looking over their shoulders at the relegation zone.
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