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The Red Devils failed to secure their place in Europe's elite club competition after finishing fifth in the Premier League.
However, they have the chance to finish the season with a trophy when they play Crystal Palace at Wembley on Saturday.
"Qualification is not a title. A title is the FA Cup," said Van Gaal.
"It is important for the players. They can look at and hold the cup, that's an exciting moment and especially here in England because the FA Cup is a big title.
"Also for Manchester United it is a big title. I have read that the club's last FA Cup was 2004 so that's a long time ago."
Who do you think should start the FA Cup final? Step into Louis van Gaals shoes and pick your XI - and then share it with your friends using our brand new team selector.
Van Gaal's future remains uncertain heading into the game.
The Dutchman said after Tuesday's final league game against Bournemouth that he planned to see out the remainder of his three-year contract at Old Trafford.
Former Chelsea boss Jose Mourinho continues to be linked with Manchester United.
Van Gaal, though, was keen not to be drawn on speculation about his future, saying his focus is only on winning Manchester United's first piece of silverware since Sir Alex Ferguson retired in 2013.
"A club like Manchester United needs silverware," he added.
"It is not good to speak about my interest. For the club it is important and it is very important for the players and, of course, for the manager and staff it is very important. But first place is the club.
"The most interesting point is that we are able to win something. We are very close but when you don't win, you have nothing.
"First we have to win, then you can say it's the first title after the period of Alex Ferguson."
Who do you think should start the FA Cup final? Step into Alan Pardews shoes and pick your XI - and then share it with your friends using our brand new team selector.
Manchester United boss Louis van Gaal believes winning the FA Cup will be more memorable for his players than qualifying for the Champions League.