The Scot hit 237 goals in 404 games for the Reds, placing him third on the club's all-time list of scorers, and won the Ballon d'Or in 1964.
He said on Thursday:
“I am at the point where I feel I want to be open about my condition. I have been diagnosed with ‘mixed dementia’, which is more than one type of dementia, in my case, this being Alzheimer's and Vascular dementia. This has been an extremely difficult year for everyone and the long periods of isolation have certainly not helped.
“It is an incredibly challenging and problematic disease and I have witnessed many friends go through this.