Manchester United, where he was in charge in 2014-2016, were among those wishing the Dutchman a speedy recovery.
“Everybody at Manchester United is fully behind our former manager, Louis van Gaal, in his battle against cancer. Sending you strength and courage, Louis,” the English giants tweeted.
Barcelona, where he was twice manager and twice won La Liga, said on Twitter: “Our thoughts and prayers go out to former Barca manager Louis van Gaal… Stay strong, Louis, and get well soon.”
Another of his former clubs, Bayern Munich, which he managed between 2009 and 2011, said: “FC Bayern would like to wish former head coach Louis van Gaal all the best as he battles cancer. We all know how strong a person he is. Hoping for a full and speedy recovery!”
Former England striker Gary Lineker, now a TV presenter in Britain, also tweeted a message of support.
“Sending best wishes and hopes for a full recovery to Louis Van Gaal,” he wrote.
Van Gaal told RTL television on Sunday that the players in the national squad “don’t know” that he has cancer.
He said he had undergone 25 bouts of radiotherapy, including during training camps with the national team.
“In each period during my time as manager of the national team I had to leave in the night to go to the hospital without the players finding it out until now. While thinking I was in good health, but I’m not.”
Van Gaal, who has also managed Ajax, said in an interview marking the upcoming release of a film about his life, entitled LOUIS, that cancer was “part of my life”.
“I’ve been through so much in my life, sickness and death, I’ve probably become richer as a person because of all those experiences.
“We do not die of prostate cancer, in most cases. Most often, these are underlying diseases that can be fatal in this form of cancer,” he added.
Last week, it was announced that Van Gaal had tested positive for Covid. In 2021, he broke his hip when he fell off his bike.
Van Gaal, in his third term as head coach of the Dutch national side, sparked controversy last month when he described the decision to hold the World Cup in Qatar “ridiculous”, accusing FIFA of being motivated by “money and commercial interests”.
The Netherlands were placed in Group A with hosts Qatar, Ecuador and Senegal in Friday’s draw.
Van Gaal returned to the Dutch national team after the resignation of Frank de Boer in the aftermath of the country’s last-16 exit from Euro 2020.
The no-nonsense Van Gaal managed the Oranje between 2000-2002 and 2012-2014, when they finished third at the World Cup in Brazil.
“Dutch football has always been close to my heart and national coaching is in my view a key position for the further advancement of our football. Moreover, I consider it an honour to coach the Dutch national team,” Van Gaal said at the time of his return.
During Van Gaal’s first spell as manager the Dutch failed to qualify for the 2002 World Cup in Japan and South Korea.
Louis van Gaal said his greatest achievement was winning the FA Cup with Manchester United
He later went on to take up a coaching job with Manchester United, but was unceremoniously axed by the club two days after winning the FA Cup in 2016.
Van Gaal at the time said winning the FA Cup with the Red Devils “was the greatest achievement of my career”.
The often brash and straight-talking Van Gaal announced his “retirement” in January 2017.
At Barcelona, from 1997-2000, he won two league titles. He returned to the Spanish giants in 2002 but an unsuccessful stay saw him quit early the following year.
By Garrin Lambley © Agence France-Presse