He was diagnosed with the condition seven years ago and now struggles to speak, his wife Liz said.
She told how she and her husband of 53 years are now living “day by day”.
Mrs McNeill said: “It is sad. We don’t know what he can remember because he can’t communicate.”
The former Celtic captain enjoyed a glittering career at the club, where he became the first Briton to lift the European Cup after a 2-1 win over Inter Milan in Lisbon in May 1967.
That team came to be known as the Lisbon Lions. 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 near Glasgow.
Mrs McNeill revealed: “His concentration is not as good as it was and he now can’t communicate very well.
“It has 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.”
The McNeills decided to bring their experiences into the open as the 50th anniversary of the Lisbon Lions’ historic victory approaches.