Ireland manager Martin O’Neill says Liam Miller’s illness "puts things in perspective".
The 36-year-old former Ireland, Celtic and Manchester United midfielder is battling pancreatic cancer.
"All thoughts go out to him and his family, [it] put things in perspective," O’Neill told reporters at a press conference ahead of the World Cup play-off against Denmark.
"I gave him his debut at Celtic. He did wonderfully well in a Champions League game one evening, he was absolutely outstanding.
"We heard about the news recently but we didn't know how bad it was. All thoughts go out to him and his family, I think he has three children as well.
Captain David Meyler echoed his manager’s sentiments.
The Hull midfielder said: "Obviously we heard the news, we're unsure about the details, but our thoughts go out to him and his family.
"We're just thinking about him and we just hope he can pull through and he's strong. That's from the whole team and everyone."
Earlier, former team-mates showed their support for Miller on social media.
Such sad news about my old Room mate Liam Miller, Pray to God he pulls through this battle with Cancer, my thoughts and prayers are with him and his family. ❤️❤️❤️
— Noel Hunt (@Noelhunt20) November 12, 2017
All my thoughts and love with Liam Miller and his family ❤️
— Darren O'Dea (@odea_darren) November 12, 2017
Celtic and Manchester United have also expressed their support for their former midfielder.
Miller is reportedly receiving treatment in the United States, where he last played for Wilmington Hammerheads.
Celtic wrote on their official Twitter account: "The thoughts and prayers of everyone at Celtic Football Club are with Liam Miller and his family at this time."
Miller made his professional debut for Celtic in 2000 after coming through the youth ranks at Parkhead and helped them win the Scottish Premier League title in 2004.
He then moved south to United on a free transfer, making 22 appearances under Alex Ferguson in two seasons - which included a spell on loan at Leeds - before joining Sunderland in 2006.
Miller was part of the Black Cats' squad that won promotion to the Premier League in 2007 but he was on the move again in 2009, this time to QPR.
A short spell at Loftus Road was followed by stints at Hibernian, Australian clubs Perth Glory, Brisbane Roar and Melbourne City, home-town club Cork City and, most recently, Wilmington.
John Hartson, who played alongside Miller at Celtic, tweeted: "My thoughts are with Liam miller and his family this morning. Hope he can get through this difficult time of his life."
Jacob Burns, the former Leeds and Australia midfielder who is now head of football at Perth Glory, tweeted: "In shock! Thoughts and prayers are with my old team Mate and friend Liam Miller as he and his beautiful family fight through this battle!"
