His former Hibs team-mate McManus also claimed that he is undergoing chemotherapy.
McManus added that Miller had been in the USA for treatment earlier this month.
Miller made 44 appearances for Celtic, scoring five goals.
In recent years the Cork-born player has plied his trade in Australia, the Republic of Ireland and the United States. Alex Ferguson and signed for Manchester United.
Speaking at his pre-match press conference ahead of Ireland's World Cup play-off second-leg clash with Denmark, the former Bhoys boss said: "I gave Liam his debut at Celtic".
"I heard about it just recently but didn't know how bad it was".
McManus added: 'He starts chemo tomorrow and is fighting hard really can't go into any more detail than that out of respect for him and family.
Sunderland, Celtic and Manchester United have expressed their support for former midfielder Liam Miller - who's understood to have pancreatic cancer.
Miller then joined League of Ireland outfit Cork City in 2015 before making the move to America to play for third-tier North Carolina-based side Wilmington Hammerheads in February 2016.
John Hartson, who himself was diagnosed with testicular cancer, also wished Liam the best with his treatment.
"Thoughts and prayers are with my old team Mate and friend Liam Miller as he and his lovely family fight through this battle!"
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