Sunderland claimed the advantage in the first leg two weeks ago but Valencia says United are ready to turn it around at Old Trafford.
"Not only are we hopeful, we have the confidence and the motivation to ensure our fans will be enjoying a trip to Wembley," the winger told MUTV.
"Our fans will be supporting us as usual, which is very important. Alongside that, we need to go out with the attitude that we always have at home: to win the game.
"For the new guys and coaching staff here, getting through would be a great confidence boost to keep working as we have been.
"It’s not about getting three points in the league; it’s about getting through to a final. Let’s hope we can do that as it’s very important to us."