Olivier Giroud believes Chelsea allowed their intensity to drop in the second half of Thursday's 4-3 Europa League win over Slavia Prague and that allowed the visitors to get back into the contest.
The Blues produced a brilliant first-half performance, finding themselves 4-1 up at the break thanks to a Pedro brace, Giroud's strike and a comical Simon Deli own goal.
But Petr Sevcik scored two fine long-range efforts early in the second period, causing the atmosphere at Stamford Bridge to become extremely tetchy towards the end.
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Chelsea ultimately did enough to advance 5-3 on aggregate, but their second-half display will have alarm bells ringing and undoubtedly give semi-final opponents Eintracht Frankfurt reason for optimism.