Celtic boss Neil Lennon is "hoping and praying" for the recovery of former Hoops team-mate Jackie McNamara.
The 46-year-old was taken to Hull Royal Infirmary on Sunday night after collapsing outside his home in York and is said to be in a critical but stable condition.
Lennon, who played alongside McNamara during the early 2000s, said: "I am lost for words, I don't know what to say.
"I spoke to him on Tuesday so I am devastated by the news and hoping he pulls through.
"People have been keeping me abreast of things. He is a strong guy, a great guy and a fit boy for someone of his age.
"He has been fit all his life so I am hoping and praying he comes through okay."
A spokeswoman for Hull University Teaching Hospitals NHS Trust said: "We can confirm Jackie McNamara was admitted to Hull Royal Infirmary on Sunday night and is currently being cared for in our Intensive Care Unit. He remains in a critical but stable condition."
“We are supporting both Jackie and his family, who request their privacy to be respected at this time.”
McNamara, husband of Samantha (49) and dad to Erin (22), Sidney (15) and Evie (14) was rushed to hospital in York before being transferred to hospital in Hull.
Daughter Erin took over his Twitter account on Monday night to thank worried fans for their support, and asked for privacy in the coming days.
Lennon is just one of several former team-mates of McNamara who have sent their best wishes since news of the former defender’s serious illness emerged.
Simon Donnelly, McNamara’s former Celtic team-mate, managerial assistant and co-owner of a pub in Spain, simply posted images of praying hands, hearts and a strong arm to show his thoughts.
One of their former Celtic comrades, Alan Stubbs, said: “Simon went away for the weekend to the Isle of Arran. He’s been trying to get back ever since he found out but the ferries have been off because of the weather.
“He’s been beside himself. It’s such a worrying time for everybody but obviously Jackie and Sid have always been the best of mates. The pair have been pretty much inseparable over the years.
“Everyone is worried sick and all our thoughts are with Jackie’s wife Sam and the kids.
“I found out on Saturday when I received a phone call from a former team-mate and my first reaction was obviously shock. I’ve been in a state of shock.
“It was the last thing I thought I was going to hear when the phone rang and it’s been hard to think about anything else.
“All you can do is hope that the operation is a real success and that he comes through it fine and makes a full recovery.”
Parkhead hero Henrik Larsson, who played alongside McNamara for the Hoops for seven years, posted a picture of his old team-mate on Instagram with the message: “He needs all the love we can give him. @jackiemac4 keep on fighting.”
Ex-Rangers manager and striker Ally McCoist, a former Scotland team-mate, posted on his Instagram account: “Rooting for you @jackiemac4, always a battler. With you and your family all the way.”
Another former team-mate, Paul Lambert, added: “It’s just horrific. A great guy and a great player. I realise it’s very easy to say these things at times like this but it’s absolutely true where Jackie is concerned. You always knew you were in safe hands when Jackie was at your back.”