Fellow former United player Danny Higginbotham said: "If true very sad news about one of my former teammates Liam Miller".
"It puts things in perspective anyway".
Miller progressed through the youth ranks at Celtic before being handed his league debut in May 2000 - a month before O'Neill took over as manager. Later playing in Australia, he then moved to home town club Cork City, before finishing his career in America with semi-professional club, Wilmington Hammerheads.
Miller played on Wearside for three years from 2006 to 2009.
Many of his former clubs and colleagues have publicly expressed their support for Miller through social media.
He tweeted: "Awful news about Liam Miller keep the fingers crossed that he somehow pulls through. Hope he can get through this hard time of his life", tweeted the Welshman.
It has been reported that the former Republic of Ireland worldwide is fighting pancreatic cancer, although this is unconfirmed.
"That's from the whole team and everyone".
"Thoughts and prayers are with my old team Mate and friend Liam Miller as he and his handsome family fight through this battle!"