The 54-year-old Australian, who became director of the Department for Children, Education, Lifelong Learning and Skills (DCELLS) in 2006, was said to be undergoing surgery this week.
His colleagues in the Ontario provincial government, where he is deputy education minister, have been told to expect Mr Marshall to be off work for a considerable time.
Messages of support were sent to Mr Marshall and his family from Wales this week.
David Egan, professor of education at the University of Wales Institute, Cardiff, said: "No one knows what the prognosis is yet and he is not taking calls. "I'm sure all his former colleagues and friends in Wales will be wishing him well at this time."
In his brief stint at DCELLS, Mr Marshall shook up the department and championed reforms, including the performance-boosting school effectiveness programme.
He endeared himself to the teaching profession with his visionary policies, along with his comical attempts at speaking Welsh with an Australian twang during conferences and meetings.
In a statement almost a year ago for TES Cymru, in which Mr Marshall announced his departure, he said his decision to leave Wales was one of the most difficult he had ever made. "I hope the new department ... will be a stronger and more outwardly focused organisation," he said.