The Red Devils must overturn a two-goal deficit from the first leg but are still reeling from Sunday's 3-0 humiliation at home to bitter rivals Liverpool
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Writing on his official Facebook page, Rooney thanked United supporters for sticking by the team in spite of the defeat and called on them to make their voices heard again on Wednesday.
"[Sunday] was one of the worst days I've had in football and was hard to take," he admitted.
"Having said that the fans were brilliant right until the end under difficult circumstances so I would like to thank them for that.
"We need to move on as we have a big game on Wednesday, if the fans can repeat an atmosphere like Sunday then we will have a good chance of going through. Thanks again for all your support."
Defeat to Liverpool has all but ended United's hopes of finishing in the top four, meaning the team will have to win the Champions League to ensure participation next season.