Believed to be back home in Cork receiving treatment, former Hibs player Tam McManus tweeted that Miller: "was in the United States of America for treatment this month and flies back tomorrow to start chemo here".
"The news - we heard about it just recently, we didn't know how bad it was - and obviously all thoughts go out to him and his family".
The 36-year-old spent two years at Old Trafford having been signed by Sir Alex Ferguson back in 2004, playing 22 games in all competitions for the Red Devils.
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.
Miller retired from playing past year and had returned to Ireland with his wife and three children to complete his UEFA coaching qualifications. We are thinking of him and we hope he can pull through and he's strong anyway.
He also has 21 senior worldwide caps, and one goal, for the Republic of Ireland.
The former Welsh global said: "My thoughts are with Liam Miller and his family this morning".
Ex-Ireland team mate Noel Hunt tweeted: "Such sad news about my old room mate Liam Miller".
The news was revealed by his former Hibs colleague Tam McManus, who also said Miller is due to start chemotherapy.
Speaking at his press conference this morning ahead of the second leg of Ireland's World Cup play-off against Denmark, O'Neill offered his support to the former midfielder.