Chelsea's Champions League campaign hangs in the balance after a night of frustration at Stamford Bridge.
We fell behind midway through the first half to a Wayne Rooney strike, as we had done a month earlier in the Premier League, but on this occasion could not find a way back into the game despite a series of chances and a major penalty shout in injury time.
The defeat means we must go to Old Trafford next Tuesday and win, though that is a feat we achieved last season in the league, and is not beyond us once more.
On the night United had goalkeeper Edwin van der Sar to thank for their advantage, the goalkeeper making save after save, in particular from Didier Drogba and Fernando Torres in either half, and when he was beaten Drogba hit the post and Frank Lampard saw his shot cleared off the line, but the main talking point will be the failure of the referee to award Chelsea a penalty after 91 minutes.
Ramires went down under a clumsy and desperate challenge from United's Patrice Evra, and a packed Stamford Bridge awaited a whistle from Alberto Undiano Mallenco. It did not come, meaning we must now triumph in the north-west next week.
It was supposed to be a night of celebration, Lampard's 500th appearance in a Chelsea shirt drawing plenty of attention before the game, while this was also Chelsea's, and Carlo Ancelotti's 100th game in the Champions League proper, the manager having taken charge of AC Milan 85 times before arriving in west London.
For the occasion the Italian recalled Torres to his line-up at the expense of Nicolas Anelka, while Yury Zhirkov was preferred to Florent Malouda on the left of midfield and Branislav Ivanovic replaced the cup-tied David Luiz in the heart of defence alongside England's new captain John Terry.
England's previous captain, Rio Ferdinand, was making his comeback for United after two months out with a calf injury. Their other main team news was that Javier Hernandez joined Rooney in attack, while Nani was left on the bench, Antonio Valencia and Ji-Sung Park Sir Alex Ferguson's preferred wide men.
Inside 90 seconds it was Chelsea gaining joy from the flanks, Jose Bosingwa scampering down the right and crossing dangerously towards Torres, Nemanja Vidic just getting a headed clearance in before the Spaniard's arrival.
Five minutes later Torres did manage a shot, powering low into the arms of Van der Sar after Patrice Evra had gifted him possession.
A quarter of an hour in United's Ji-Sung Park saw his effort deflected behind for a corner but the majority of action was in the middle third, each side sizing the other up like a pair of boxers in the early rounds. It would be a tense evening.
Quick thinking from Lampard and Ramires earned Drogba a shooting chance for which he needed little encouragement, unleashing a powerful drive Van der Sar did well to tip over into the Shed End.
United were yet to register a shot on target of their own, but their first would be devastating. It came on 23 minutes after Rafael had brought the ball out of defence and fed Michael Carrick. The midfielder's cross-field pass picked out Ryan Giggs, who was able to knock the ball past Bosingwa in one touch and pull back for Rooney, who had peeled away from his marker to side-foot in off the post.
The goal game at a similar time to Rooney's strike here in the Premier League at the start of March, and was the only truly memorable action of a first half-hour punctuated by the Spanish referee's regular whistle.
Rooney bent a late strike towards Petr Cech's goal that the Chelsea man gathered with ease, and then just before half-time Chelsea should have gone level.
Drogba worked space for himself to shoot, the effort deflected onto the post before Lampard followed up, skewing his left-footed volley slightly, but still on target, only for Patrice Evra to clear a certain goal off the line.
The second period began with further hold-up, Rafael twice requiring treatment, either side of a gilt-edged opportunity for Ramires. Evra tackled himself out on the touchline allowing Drogba the ball, and the Ivorian crossed early for the incoming midfielder who should have hit the target but instead headed wide of the far post.
Rafael could not continue and was replaced by Nani, attack-minded Valencia moving deeper to right-back to accommodate the Portuguese. Rooney too was dropping deeper when necessity called, pressuring Lampard and Michael Essien when they had the ball.
Drogba flashed an overhead kick across the United area on 56 minutes, and 10 minutes later Cech had to push an awkward Nani cross away from a lurking Hernandez. Essien volleyed just wide from distance. Clear cut chances were so far unavailable for the Blues.
Sensing the tie slipping away with United looking relatively comfortable in dealing with his side's threat, Ancelotti made two changes. Malouda replaced Zhirkov, Anelka came on for Drogba, the formation stayed the same.
Within four minutes both subs had contributed to create a chance. Malouda won a header, Lampard flicked on to Anelka, whose loose touch allowed Evra to challenge, the ball falling to Bosingwa, whose deep back-post cross was met by a superb Torres header that few keepers would save. Unfortunately one who would is Van der Sar, whose long reach allowed him to claw the ball away and preserve his side's lead.
One last change for Chelsea. Mikel on to hold in midfield, Essien moved to right-back and Bosingwa withdrawn. United threw on Berbatov for Hernandez. Thirteen minutes remained.
A curling Lampard free-kick from wide evaded everybody, narrowly missing the target, before the same player fired hard and low from a central position, straight at the goalkeeper.
Time was ticking away, though it must be remembered only towards half-time in the tie, but Chelsea were playing with urgency. Ivanovic strode forward, beat two challenges and shot low, but again too straight and Van der Sar made his seventh save of the night, by far the busier stopper.
Cech did have work to do though, dispossessing Nani at his feet inside the area, before Anelka arrived just too late to convert a cross at the other end. His low shot on 89 minutes had the keeper scampering, successfully, to save, and there would be four added minutes.
One minute past the 90 came a major talking point. Chelsea had been here before with Barcelona in 2009, with Inter in 2010.
Ramires raced through to Torres's flick, Evra lunged, made contact, and brought the Chelsea man crashing to the floor. Penalty, surely? Not in Europe, it seems. Play on, waved Mr Mallenco.
No further opportunities came, but that challenge will live long in the memory. At Old Trafford in six days' time, Chelsea will be hoping it does not matter. Drogba and Joe Cole scored there last season, who will be the hero this time around?
Chelsea (4-4-2): Cech; Bosingwa (Mikel 77), Ivanovic, Terry (c), Cole; Ramires, Essien, Lampard, Zhirkov (Malouda 69); Drogba (Anelka 69), Torres.
Unused subs: Turnbull, Ferreira, Mikel, Benayoun, Kalou.
