United must overturn a two-goal deficit from the first match in order to reach the quarter-finals but are still reeling from Sunday's 3-0 humiliation by bitter rivals Liverpool at Old Trafford.
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"[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 they will have to win the Champions League to ensure their participation next season.