Celtic boss Neil Lennon says he is "hoping and praying" for the recovery of former Hoops team-mate Jackie McNamara.
McNamara, 46, was taken to Hull Royal Infirmary on Sunday night.
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 OK."
A post on McNamara's personal Twitter account on Monday, read: "This is Jackie's daughter. Thank you all for your kind words and support.
"My dad is getting the best care possible, as you all can understand this is difficult for us as a family and request privacy at this time."
Celtic wrote on Twitter: "The thoughts and prayers of everyone at #CelticFC are with Jackie and his family."
A statement on the Scotland National Team Twitter account read: "The thoughts of everyone at the Scottish FA are with former Scotland player Jackie McNamara and his family."