McNamara, 46, is understood to have been admitted to hospital for brain surgery.
A post on McNamara’s personal Twitter account 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.”
John Hartson, his former Celtic team-mate, stated that McNamara had collapsed on Saturday.
“Worried sick for my former teammate and great friend Jackie McNamara who collapsed at home Saturday with a bleed on the brain… My thoughts are with his wife Samantha and the children,” Hartson tweeted.
Scottish champions Celtic wrote on Twitter: “The thoughts and prayers of everyone at #CelticFC are with Jackie and his family.”
McNamara, who claimed four league titles at Celtic and won 33 Scotland caps, also played for Wolves, Aberdeen, Falkirk and Partick Thistle.
He has managed Partick, Dundee United and York City.
