Rafael Benitez remains confident his Chelsea side will advance to the Europa League quarter-finals by overturning their 1-0 last-16 first-leg defeat to Steaua Bucharest.
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In the first of two games in four days and the Blues' 50th game of a turbulent season, last season's Champions League winners were beaten after Ryan Bertrand was penalised for pulling back Raul Rusescu, who converted a 34th-minute spot-kick.
Blues interim manager Benitez described the penalty award as "soft" but Chelsea were lacklustre, offering little attacking threat, and take a deficit into next week's second leg at Stamford Bridge.
"We know that we have to do our job now at home," said Benitez, who must now take his side to Manchester United for Sunday's FA Cup quarter-final.
"In these kind of games and this kind of competition, if you don't score away it's always more difficult. Still you have to believe. "We are disappointed, but still we have the belief we will win at home."
The result in the Romanian capital will only serve to increase the pressure on interim manager Benitez.
Chelsea began the season in contention for seven trophies and the Europa League is the unexpected eighth after their embarrassing Champions League group stage exit which prompted Benitez's unpopular appointment.
Two chances of silverware remain, but chief executive Ron Gourlay this week insisted a top-four place in the Barclays Premier League and Champions League football next term is the priority.
It is just as well as now they must beat United on Sunday and overcome an impressive Steaua, 12 points clear in the Romanian league and conquerors of Ajax in the last round, to keep their two trophy hopes alive.
The decisive moment came when left-back Iasmin Latovlevici, earmarked as a dangerman by Benitez on the eve of the match, crossed with the outside of his right foot towards Rusescu, whose attempts to reach the ball were thwarted by a pull from Bertrand.
Referee Sergey Karasev booked the left-back, playing in place of Ashley Cole and exposed at the far post, and Rusescu sent the resulting spot-kick under Petr Cech.
"I think it was very soft," Benitez told ESPN. "To give a penalty in this situation, and especially when you have so many referees in their positions, it is soft.
"You cannot change the situation but it is very soft.
"If you give the penalty, why not the (red) card? Strange."
Just like United, Chelsea warmed up for the FA Cup clash with a European defeat, but at least their rivals were defeated by Real Madrid. Benitez made five changes from the side which beat West Brom, with John Terry included in central defence.
Given the captain's fitness travails, it is likely he will not be in the first XI at Old Trafford.
Benitez insisted the looming contest at Old Trafford had nothing to do with the loss.
He said: "It's another competition and atmosphere. It will be totally different. It's another challenge for us."
Steaua were spurred on by a vociferous and passionate 55,000 crowd, which booed every Chelsea touch and loudly roared their own team on.
Steaua boss Laurentiu Reghencampf said: "Tonight we only wanted to score and to stop Chelsea from scoring. This is just the result.
"Regardless of what will happen in London, they have to be congratulated for winning against a very, very good team.
"This game is played in two games. We have to perform even better than tonight in London in order to qualify for the quarter-finals."