The all-English tie is the most delicately balanced of the four quarter-finals after United won 1-0 in London last week thanks to a Wayne Rooney strike. But a raft of statistics and the valuable away goal tip the tie in the home side's favour.
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By contrast, Tuesday's other match looks a mere formality with Barcelona holding a 5-1 advantage over Shakhtar Donetsk ahead of their second leg in Ukraine.
Chelsea can take comfort from the fact they were the last team to win at Old Trafford, a 2-1 victory last April playing a big part in their triumphant Premier League title surge.
"We have a good memory of that but this is a different story," manager Carlo Ancelotti told a news conference.
"They (the players) have to truly believe we can win there. I don't know if they will be able to do this."
Only two sides have recovered from a first-leg home defeat to win a Champions League knockout tie and if Chelsea become the third they would find it particularly sweet after losing the 2008 final to United on penalties.
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