Legendary Bayern Munich and Germany striker is suffering from Alzheimer's disease, his former club have confirmed.
Bayern announced the news in a release via their official website as they paid tribute to Muller ahead of his 70th birthday later this year.
He said: "Gerd Muller is one of the greats of world football. Without his goals, FC Bayern and German football would not be where they are today.
"Despite his success, he was always modest, and that's what really impressed me. He was a wonderful player and a friend.
"He brought experience as a coach and helped to create world champions in Philipp Lahm, Bastian Schweinsteiger and Thomas Muller."
Rummenigge also asked for Muller's family to be given privacy over the coming weeks.
"This is not only the express wish of his family, but also FC Bayern," he added. "Bayern will always support Gerd Muller and his family whenever it is necessary."
Muller's doctor, Professor Dr Hans Forstl, thanked the club for the support given so far.
"Despite unmistakable signs of his illness, he was treated with great sympathy and respect by the Bayern family, fans and the media," he said.
"That was very important because it is a wish for every Alzheimer's patient to stay in the environment in which they feel comfortable for as long as possible."
