Toshack scored nearly 100 goals and won multiple trophies at Anfield, where he formed a lethal strike partnership with Kevin Keegan in the 1970s, and he earned 40 caps for Wales.
Among the teams he coached during a long managerial career were Real Sociedad, Real Madrid and Wales.
The 77-year-old's son, Cameron, who works as a football coach in Thailand, spoke to the Daily Mail about Toshack's illness.
"It's a terrible disease," he said. "It's the short-term memory where we're seeing it - I speak to him most days and if we chat in the afternoon, he might not remember that we also spoke in the morning.
"But if I ask him about the Liverpool days, or Sociedad or Madrid, the detail is amazing.
"The other day he was telling me about a Real Madrid game against Arrigo Sacchi's AC Milan and exactly how he tweaked his midfield to deal with Marco van Basten.
"The game could have been yesterday, his memory was so clear."
Toshack won the Copa del Rey during one of three spells at Real Sociedad and twice managed Real Madrid, winning La Liga in 1990.
He also coached in France, Turkey, Morocco, Azerbaijan and Italy and had two stints as Wales boss.