Was in the US for treatment this month and [flies] back tomorrow to start chemo here.
The Cork native, now 36, is reportedly flying back to Ireland on Monday, where he will begin chemotherapy after undergoing initial treatment in the US.
Very sad news about one of my old teammates Liam Miller, thinking of him and his family at this very hard time.
"I heard recently but I didn't know how bad it was", he said. We're unsure of all the details, but our thoughts go to his family.
Miller played on Wearside for three years from 2006 to 2009.
Liam Miller, 36, was sent messages of support from former team mates and fans in his battle against the disease.
He said: "Awful news about Liam Miller, keep the fingers crossed that he somehow pulls through".
For Ireland, Miller earned 21 senior caps under Brian Kerr, Steve Staunton and Giovanni Trappatoni.
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.
"Thoughts and prayers are with my old team Mate and friend Liam Miller as he and his attractive family fight through this battle!"