Former Celtic captain Jackie McNamara is in hospital after suffering from “bleeding on the brain”.
The 46-year-old, who also played for Scotland and Dunfermline during a top-flight career, collapsed at home in York on Saturday.
Former team-mate John Hartson, who played with McNamara for four years between 2001 and 2005, said he was “worried sick”.
He added: “My thoughts are with his wife Samantha and the children.”
McNamara’s daughter has requested privacy for the family and said he is getting the “best care possible.”
Tweeting from her father’s account, she wrote: “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.”
Former right-back McNamara, whose father Jackie snr also played for Celtic in the 1970s, was signed by Tommy Burns in 1995.
He went on to lift four league titles, three Scottish Cups and three League Cups during a ten-year spell at Parkhead.
He lifted the Scottish Cup as club captain in 2004 and 2005.
After retiring from playing he was appointed full-time manager at Partick Thistle who he helped gain promotion to the Premier League before leaving for Dundee United.
At Tannadice he took the club to two consecutive cup finals, losing the 2014 Scottish Cup to St Johnstone and the following season’s League Cup to Celtic.
Despite a good start life at United soon turned sour for McNamara and he would leave the post in late 2015 before being hired by York City around a month later.
Since leaving York he returned to first club Dunfermline on a consultancy role and launched a Spanish pub chain with friend and former Celtic team-mate Simon Donnelly.