If you go away from home and lose 2-1 in the first leg of a major quarter-final, you consider that a good result because destiny is in your hands.
All United have to do is beat them at home and keep a clean sheet and the job's done.
Unfortunately, though, keeping a clean sheet has not proved easy for United of late.
They've had several injuries which means they're a bit thin defensively and goalkeeper Edwin van der Sar has started to make a few mistakes.
Add to that Roma's clever attacking play in the first leg and you have a very difficult game for United.
It's all about strong minds and playing as a unit defensively because at this level, any mistake you make is likely to be punished.
But if I was a betting man, I'd have my money on United to go through.
Jose Mourinho was quite positive about Chelsea's 1-1 draw at home in the first leg but I think the Blues are in a precarious position.
At home, you're looking for a win and a clean sheet and Chelsea didn't get either.
However, the position Valencia find themselves in is a tricky one because all they need is a 0-0 draw and they're through.
That presents a dilemma: do they play for 0-0 or go for the win?
How many times do you see sides in that position get caught between the two things and end up going out?
And the thing about Chelsea is they are so robust. Whatever the situation, they just keep on going.
So while I think I'd rather be in Valencia's position, I would never count out Chelsea pulling through.
But I think Liverpool will just kill it so it's a nothing match and Rafael Benitez's side can look forward to playing the winner of the Valencia-Chelsea quarter-final.
Liverpool have high hopes of going all the way in this competition and you can see why. For starters, they are a much better side than they were when they won it two years ago.
They've been playing well in big games at home. They may have lost to Manchester United and Barcelona but they played exceptionally well both times and if they play like that at Anfield in the semi-finals, they'll be happy.
Their quality players have really gelled, they're starting to score goals and they're keeping clean sheets. It's a potent combination.
What a first leg it was: plenty of goals and incidents.
With those two away goals, you'd expect Bayern to go through which I think is bad news for Manchester United should they progress.
I think Sir Alex Ferguson would rather be up against AC Milan but we'll have to wait and see.
You expect Bayern to go through - the first leg was great, full of goals and incidents. I think if Man Utd were to go through they'd rather play Milan, but we'll wait and see.