Sunday evening, the 70-year-old made the admission while promoting a new documentary about his life on the Dutch television show, Humberto.
“They have no idea. When they see a flush on my cheeks, they think, ‘what a healthy (person) that is.’ Of course, this is not the case,” Van Gaal said of his team.
“I’ve been through a lot with illnesses, including with my own wife,” he added. That’s just a fact of life.
“(I) as a human being have most likely become richer as a result of all those experiences. I thought that should be in the film as well.”
Van Gaal will take over as manager of the Netherlands for the third time in August 2021, having previously led them to a third-place finish at the 2014 World Cup.
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He took over for Frank de Boer following a disappointing Euro 2020 campaign in which the Dutch were eliminated in the last-16, and he is now set to lead his country at the 2022 World Cup in Qatar later this year.
They were drawn in Group A of the tournament, which begins in November, against hosts Qatar, Africa Cup of Nations champions Senegal, and Ecuador on Friday.
Van Gaal was the manager of Manchester United between 2014 and 2016, before returning to the Netherlands.
Despite leading the club to FA Cup victory in his final game, he was fired and replaced by Jose Mourinho.
On their Twitter feed, United posted a message of support that read: “Everyone at Manchester United is rooting for our former manager, Louis van Gaal, as he battles cancer. Louis, I’m sending you strength and courage.”
Marcus Rashford, a Manchester United forward, also tweeted his support for his former manager, writing: “LVG gave me my debut. I will be eternally grateful. You can overcome this. We’re all rooting for you.”
Van Gaal previously won league titles with Ajax, with whom he also won the Champions League in 1995, Barcelona, AZ Alkmaar, and Bayern Munich.