United hold the whip hand following Wayne Rooney's goal in their 1-0 quarter-final first-leg win at Stamford Bridge and in most circumstances in the Champions League and for United, that would be that.
Sir Alex Ferguson's team have lost one of their last 30 matches in the competition at Old Trafford and none in the last six years. They have progressed in all 16 European ties where they won the first leg away from home.
Only two of the teams' last 12 competitive games have been decided by more than one goal so both sides know the Londoners are still very much in the tie.
"Of course Chelsea can do to us at Old Trafford what we've done there," said Ferguson. "We've only got a one-goal lead and we have to be very careful and give a top performance.
"It will be a tough game. The tie is not done."
Ferguson had the luxury of making eight changes from the first leg for Saturday's Premier League game against Fulham and those on duty came through unscathed - and barely out of breath - after a straightforward 2-0 win.
"Obviously we know it will be difficult to win there but we have the confidence to do it, we did it last year," said manager Carlo Ancelotti.
"We are at a disadvantage now but we played well (in the first leg), we should have had a penalty, we hit the post."
Ancelotti's biggest headache is how to best use his trio of front men - Didier Drogba, Nicolas Anelka and Fernando Torres, who took his goal drought to 10 matches after coming off the bench against Wigan.
