Chelsea manager Andre Villas-Boas implored his side to "step up" after their Champions League last 16 hopes were left in the balance by a late 2-1 defeat at Bayer Leverkusen.
Didier Drogba put Chelsea ahead in the second half, but just like they had in their previous two away games in Europe, they could not hold on to their lead.
First, substitute Eren Derdiyok nodded home an equaliser before Manuel Friedrich sealed the win in stoppage time to seal the Germans' place in the knockout stages and leave Villas-Boas sweating over his future.
Chelsea need to beat Valencia at home to be sure of qualification, while a 0-0 draw will also be enough.
Villas-Boas told ITV: "Our focus must be at the moment on Valencia at home. We are still in front of Valencia but it will be a good head-to-head battle, decisive.
"It's tough for the English clubs at the moment in the Champions League but we have to face it, put on a brave face and get back to the wins. We've been suffering a lot, conceding a lot of goals, [but] we haven't conceded from a corner for a long time - I think that was the first time.
"We were compact and disciplined in set plays but one detail in the last moment [cost us]. There are no perfect organisations. One small detail makes the whole difference and in this competition you pay heavily.
"We need to get the concentration and focus right. No excuses - if we have to go one more step up that's what we have to do.''
Former Chelsea midfielder Michael Ballack backed his old side to bounce back after playing a key role in their defeat. He said: "We are in the next round, but Chelsea have a game at home against Valencia so they still have a good chance.
"Every team has sometimes a difficult moment and Chelsea is in a moment like this now. I know the players are very competitive and I think they will come back. They have a strong side and they will come back.''
Ballack admitted his side rode their luck to claim victory, adding: "It's always a little bit lucky if you score in the last minute but we believed. They are not in the best moment, but of course in the end it was a little bit lucky.
"We have a young team. We started well, but during the game I had the feeling we didn't believe 100% we could win the game. Chelsea felt it and they got stronger.
"But we got better and better and we deserved a draw - a draw would have been okay - but in the end we got the victory.''