I think the draw is all set, and it seems Liverpool won't be able to play Barcelona in the final. So now I'm going for Liverpool vs Man U.

Do they not re-draw for the semis?

Nope!
winner of man u vs porto goes against winner of arsenal villareal in the semis.
etc

If that happens, think of the irony for Thierry Henry and Alexander Hleb that would come with that.
Losing the CL-final as Arsenal players against Barcelona and then three years later losing it again, but this time as Barcelona players against Arsenal.