United host the Gunners - the current cup holders - on Monday in the quarter-finals.
And Van Gaal believes the club's top-four battle in the Premier League means there's more at stake at Old Trafford.
He said: “It’s a game between two teams of 11 players.
“United and Arsenal are placed third and fourth in the league, so it’s almost like a final, I think.
Van Gaal: Team full of confidence after Newcastle win [AMBIENT]
It's almost like a final
Louis van Gaal
"We have the support of our fans but Arsenal shall bring a lot of fans too.
"Also, Arsenal and United are now in third and fourth place, so it is a big event with two good teams. I hope we can give a fantastic match for the fans.
"I think if we beat them it is a big blow for them, but if they beat us it is a big blow for us.
"It is very important game, not only for the FA Cup but also for the rat race."
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