Ex-England striker Chris Sutton said: "All thoughts and prayers go out to my old teammate Liam Miller tonight".
"The thoughts and prayers of everyone at Celtic Football Club are with Liam Miller and his family at this time, ' read a tweet from the Scottish champions" Twitter account.
Liam Miller won the A-League premiership and championship with Brisbane Roar in 2014.
His wife is by his side and I understand they hope to bring him back home here.
Believed to be back home in Cork receiving treatment, former Hibs player Tam McManus tweeted that Miller: "was in the US for treatment this month and flies back tomorrow to start chemo here".
Pic: SNS (Miller was a highly regarded talent when he broke into the Celtic team as a teenager) Former Celtic and Manchester United midfielder Liam Miller is having chemotherapy treatment for cancer.
Later in his career, Miller would spend some years in Australia's A-League, before returning home to play with Cork City in 2015 before heading overseas again, joining the Wilmington Hammerheads in North Carolina.
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.
For Ireland, Miller earned 21 senior caps under Brian Kerr, Steve Staunton and Giovanni Trappatoni.
While Stoke playmaker Charlie Adam added: "Thoughts are with Liam Miller and his family at this tough time.#staystrong".
"The thoughts of everyone at Manchester United go out to Liam Miller and is loved ones at this hard time" a tweet from United read.
This story originally appeared in the Evening Echo.