"Until a week on Sunday, United seemed unbeatable, but after the way they fell down against Arsenal we can see that they're not invincible," said Luisao.
"This has opened up to us how we can compete with them.
"Rooney is not on form, though we can't rely on that.
"They will come at us slightly calmer after beating Celtic at home, but if they relax they will be made to pay because for us this match is like a final.
"I thought United were more dangerous when they had van Nistelrooy, but Saha is scoring goals, so our defence are prepared for a lot of work."