"Maybe it can be an influence. There is a lot of criticism. They have to use their tools to ignore it. You have to prepare yourself properly. It should not be an influencing subject," Van Gaal told reporters.
However, midfielder Juan Mata insisted he was not influenced by the criticism but he understands the frustrations of the supporters.
"I am not bothered. I was very sad and frustrated because of the result and the fans. Not with the criticism," Mata said on the eve of the second leg against Midtjylland at Old Trafford.
"We feel their frustration because it is our frustration."
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With breaking into the top-four in the league looking less likely by the week for United, Mata wants to qualify for the Champions League next season by winning the Europa League.
"Hopefully we can lift the trophy. It has a double meaning. The first time for the club and also win the Europa League to qualify for the Champions League," the Spain international said.
Van Gaal confirmed that United will be without first-choice goalkeeper David de Gea on Thursday, but long-term injury absentee Marcos Rojo is likely to feature for the first time since undergoing shoulder surgery in November.
(Reporting by Shravanth Vijayakumar in Bengaluru, editing by Ed Osmond)
