The German did not specify what kind of cancer he had.
"Unfortunately, I had to withdraw from public life in recent months, for I was diagnosed with cancer during a routine examination," Daum wrote on Instagram.
"Since then, I am in the best hands with Prof. Dr Jurgen Wolf and the team of the CIO (Centre for Integrated Oncology) Cologne and I am glad that the treatment is working very well for me!."
Daum has not had a coaching job since 2017 when he left his position as manager of Romania's national team. He also coached clubs including Eintracht Frankfurt and Turkey's Besiktas.
