Manser was acting chairman for five months earlier this year when Mackay took medical leave of absence to focus on his treatment following surgery for a brain tumour in April.
Mackay recovered sufficiently to return to his role in September, but following a recent sudden worsening of his condition, the board has agreed to his request for a further leave of absence.
SABMiller said its board will continue to keep the position under review.
(Reporting by James Davey; Editing by Brenda Goh)