So, how do our expert team of former stars expect the return tie at Old Trafford to unfold tonight?
Should Chelsea play Fernando Torres from the start?
JAMIE REDKNAPP: Torres needs to play. If he really is Carlo Ancelotti's man, then now is the time to prove it. Chelsea's big threat is their goalscorers and they should play 4-3-3. Chelsea lacked guile in the first leg. There was no one to unlock the door. Yossi Benayoun has the magic to do that and Torres enjoys playing with him. Benayoun is a decent weapon to have maybe on the bench.
MARTIN KEOWN: Torres should play and alongside Nicolas Anelka. They both have pace and that is what can trouble Nemanja Vidic and Rio Ferdinand. Picking Didier Drogba plays right into Vidic's hands. Anelka must play on the shoulder of the last defender though and not sit too deep like he did when he came on in the first leg. If he does, he and Torres can stretch the defence and pin back the full backs.
ANDY TOWNSEND: No, Torres should be on the bench. There is more pressure on him with each game and this is too important a match to be worrying about any one player in particular. Chelsea need to produce their best performance of the season and I don't think that should involve Torres. I would play Didier Drogba up on his own, with Nicolas Anelka on one side and Florent Malouda on the other.
Given the nod: Fernando Torres goes close to breaking his Chelsea duck in the first leg
Can Frank Lampard find his form and turn the tie?
REDKNAPP: Chelsea have always had success with the 4-3-3 formation; look at the trophies they've won. It suits Lampard bursting from midfield; it gives him license to get forward. The 4-4-2 formation they've used since Torres arrived simply doesn't suit Lampard. He's not out of form, but changing to 4-3-3 will make all the difference to his game.
KEOWN: Lampard has struggled since Chelsea moved to a 4-4-2 formation. He needs to be a bit cuter with his runs into the box and arrive later. People may worry about Chelsea being left short of numbers in midfield when Lampard bombs forward, but Ramires has impressed me and has enough energy to cover the ground.
TOWNSEND: Reverting to a 4-3-3 would bring the best out of Frank Lampard. He is the man for a big occasion and has a habit of scoring when it matters most. He hasn't been at his best in recent weeks but when you are looking for a goal he is right up there alongside Drogba.
Inspirational figure: Can Frank Lampard fire Chelsea to victory?
What do Chelsea need to do to get through?
REDKNAPP: Stay calm, there's only one goal in it and there's plenty of football to be played. No one in football would say it's game over already. When you have a lead just look at Rory McIlroy you don't know whether to sit on it or attack. John Terry and Branislav Ivanovic will be the key men. Wayne Rooney was the difference in the first leg he was really at it. Last week Lampard and Michael Essien weren't sure whether they were supposed to pick him up, but tonight Terry or Ivanovic have to step out and take that responsibility.
KEOWN: Torres and Anelka versus Ferdinand and Vidic will be key. Ferdinand was majestic in the first leg but not having played much recently could catch up with him. The Chelsea strikers need to pull him all over the park to really test his body. And Vidic does not like playing against pace they should exploit that. Chelsea's strikers will be irrelevant if they cannot get possession that means Michael Essien will be soimportant. He gets everywhere and can provide the passes to the front two.
TOWNSEND: If Chelsea concede, I think they will go out so Terry has to have the game of his life. He has been in superb form in recent weeks but Rooney has scored twice against him in their last two meetings and he cannot afford a repeat.
Sensational: Rio Ferdinand was outstanding at Stamford Bridge - but can his body keep up the same level of performance?
Who do you think will go through?
REDKNAPP: Manchester United but not before some more drama.
KEOWN: I think Chelsea will cause an upset and win it on the night to go through.
TOWNSEND: United. They are formidable at Old Trafford but it won't be easy.
Pressure, what pressure? Carlo says he won't be sacked if Chelsea get KO'd Martin Samuel: Dismiss United at your peril - Fergie has quietly rebuilt a formidable forceWe'll thwart Roman again! Rio's back to lock it down and prolong Chelsea waitAncelotti bidding for repeat of his ultimate Old Trafford glory night with MilanMartin Keown's Big Match Dossier: Pea is for Perfect in United's dream strikeforceThree feelgood facts for Fergie and Manchester United. and three reasons to cheer Carlo and ChelseaAll the latest Chelsea news, features and opinionAll the latest Manchester United news, features and opinion
Explore more:People: John Terry, Michael Essien, Nemanja Vidic, Martin Keown, Didier Drogba, Wayne Rooney, Rio Ferdinand, Frank Lampard, Nicolas Anelka, Jamie Redknapp, Carlo Ancelotti