Sir Alex Ferguson has extended his deepest sympathy to the family of Liam Miller who died on Friday at the age of 36.
Miller - whose former clubs included Celtic, Manchester United and Hibernian - was diagnosed with pancreatic cancer last year and had been undergoing treatment in the United States and Ireland.
Sir Alex, who signed the Republic of Ireland midfielder for United in 2004, said: "I am very sad to hear that Liam has died.
"He was a good professional, who served our club in an excellent manner. He was a quiet, unassuming young man and was popular with everyone at Carrington.
"On behalf of all my staff who worked with Liam at the time, we send our deepest sympathy to his family."
Celtic held a minute's silence in honour of Miller ahead of their Scottish Cup fifth round win over Partick Thistle.
Meanwhile, Manchester United will wear black armbands in Miller's memory for Sunday's televised clash against Newcastle at St James' Park.
The Football Association of Ireland have also confirmed there will be a minute's silence ahead of the President's Cup tie between Dundalk and Miller's hometown club Cork City on Sunday.