Real demonstrated their explosive qualities on the break during a stunning 3-1 win at Barcelona in the Copa del Rey last week, in which Cristiano Ronaldo scored twice.
And Valencia, who played the final 20 minutes of United’s 1-1 draw at the Bernabeu in the first leg three weeks ago, says the Reds will need to focus for 90 minutes to keep Jose Mourinho's men at bay tonight.
"I think it’s going to be a close match," the Ecuadorian told ManUtd.com. "Real are a team that exploits the counter-attack and we must be very cautious and focus the entire game.
"I don’t think we have a real advantage after the first leg. In these types of games no-one has the edge. It’s 90 minutes in which you need to do your best, and this is the only way you can gain an advantage.
"It’s exciting. I hope I get to play and we do a great job to send the fans home happy."