The Lisbon Lions hero was diagnosed with the condition seven years ago and now struggles to speak, his wife Liz has said.
She told how she and her husband of 53 years are living day by day, telling the Sunday Mail: “It is sad. We don’t know what he can remember because he can’t communicate.”
The former Hoops captain enjoyed a glittering career at the Parkhead club, where he became the first Briton to lift the European Cup after a 2-1 win over Inter Milan in Lisbon in 1967.
McNeill, 76, also led Celtic to nine successive league titles and won seven Scottish Cups and six League Cups, before having two spells as manager.
He is currently being cared for by Liz, 73, at his home in Newton Mearns, Glasgow.
His wife has spoken to the Sunday Mail and the Scottish Sun on Sunday newspapers about her husband’s battle with the degenerative brain illness.
She said he relies mostly on hand gestures to communicate his feelings.
She told the Sun on Sunday: “His concentration is not as good as it was and he now can’t communicate very well. It’s affected his speech over the last year or two.
“Sometimes, if something annoys him, he can still say a few words like ‘don’t do that’. But in general he finds it very difficult. It’s not because he doesn’t know how to speak. There’s just a part of his brain that won’t let him. I miss the conversation.”
The McNeills decided to bring their experiences into the open as the 50th anniversary of the Lisbon Lions’ historic victory in May 1967 approaches.
And they have backed a campaign by the Sunday Mail for more funding to further research links between dementia and heading a football.
Liz said: “I think it’s the right time for us to talk about this now. Heading the ball and the possibilities of concussive effects on the brain needs more discussion.
“We don’t know if Billy’s dementia is linked to his football. More research needs to be done.”