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United are 2-1 down to the Danish side going into Thursday's last-32 second leg, having already been knocked out of the Champions League.
Asked what he wanted from his team, Van Gaal replied: "Desire, hunger. A lot of times I use the word 'horny'."
Europa League success is United's best hope of Champions League qualification.
After losing 2-1 to Sunderland in the Premier League, the Red Devils are six points shy of the top four, with Van Gaal saying it will be "very difficult" to qualify for Europe's top club competition through the league.
The United manager, 64, continues to be dogged by speculation that he is to be replaced by ex-Chelsea boss Jose Mourinho and has also faced criticism from fans.
Mourinho has said he does not know about becoming the next Manchester United manager
"There is a lot of criticism," Van Gaal said. "The players have to use their tools to ignore it. It should not be an influencing subject."
Danish champions Midtjylland were only formed in 1999 and are in their 10th season of European football.
"Coming to Old Trafford is the match of their lives," said Van Gaal. "I was not a great player, but I was always at my best in a full stadium."
Manchester United boss Louis van Gaal has told his players to be "horny" after demanding more passion for the Europa League clash with FC Midtylland.