In 2009 the Welshman was diagnosed with cancer which had developed from testicular and had spread throughout his body.
Hartson survived that ordeal to set up a Foundation to raise awareness of the illness and provide support for people battling against it.
On Sunday night news emerged that Miller was suffering from cancer with his former Celtic team mate quick to offer his support.
“I’ll be there for him,” Hartson told The Sun. “Absolutely, I phone people every day who are going through cancer.
“I visit people in hospices. I do my utmost because I know what it can do to you.
“Liam won’t be any different. I called twice but it rang out so I left a message. I don’t know if he’s in a hospital bed or if he’s even able to talk to me.
“All I know is he’s suffering. I hope he finds the strength to get through this.
“When I was ill I had so many people supporting me. There’s no doubt he’s going to get the same support.”
Miller broke into the Celtic side at the start of the 2003/04 season on the back of the club reaching the final of the UEFA Cup in Seville.
His passing ability and forward runs made a big impression on a squad used to success with the young midfielder scoring against Lyon and Anderlecht in the Champions League.
Those performances earned him a move to Manchester United and although it never worked out as he’d hoped at Old Trafford he did earn 21 full caps for Ireland.