Noel Hunt, who represented Ireland alongside Miller, took to Twitter on Sunday, writing: "Such sad news about my old roommate Liam Miller, pray to God he pulls through this battle with cancer, my thoughts and prayers are with him and his family." One day later, Celtic and United echoed the support by saying their thoughts are with the footballer's family and loved ones.
Miller, who's reportedly undergoing treatment for pancreatic cancer in the United States, according to the Guardian, made his professional debut at Celtic in 2000, and helped the Bhoys conquer the 2003-04 Scottish Premier League.
"I gave him his debut at Celtic," Ireland manager Martin O'Neill said. "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. Such a young man."
In 2004, Miller joined United on a free transfer. He spent two seasons with the Red Devils, making 22 appearances under Sir Alex Ferguson. During that time, he was loaned out to Leeds United.
Miller last played for the Wilmington Hammerheads, a club that competes in the Premier Development League.