Strootman is expected to miss the World Cup after suffering a serious knee injury.
"What has happened to Kevin is really dramatic," van Gaal told the Dutch football federation's website. "I can imagine that at the moment his world is falling in, at a very important moment.
"He gave a great contribution to our qualification for the World Cup in Brazil.
"We have never had to do without him.
"But the person most affected is Kevin, who has seen his dream fall to pieces."
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