Chelsea goalkeeper Petr Cech has described his side's 1-0 first-leg defeat to Steaua Bucuresti as "the worst result".
The scoreline, which came about courtesy of Raul Rusescu's 34th-minute penalty, means that Chelsea will have to score at least three goals to progress if they fail to keep a clean sheet in next week's second leg at Stamford Bridge.
"We ended up with the worst result," Cech told BBC Sport. "We didn't get a goal and it is not the result you want in the first leg.
"We had the better chances. I hardly made a save and they had a penalty which they scored from.
"The most disappointing thing is that we did not score. That is the way it is and we have made it hard for the return leg but 1-0 is not a scoreline we cannot overturn."
Chelsea have kept just two clean sheets in 10 European matches this season.
