Neil Lennon described Celtic as "a club in mourning" following the death of Billy McNeill.
McNeill, who played for Celtic his whole career, was captain of the famous 'Lisbon Lions' side who won the European Cup in 1967 and he also went on to manage the club.
The Parkhead club announced the 79-year-old's death on Tuesday morning.
Hoops' interim boss Lennon said: "We are a club in mourning.
"On behalf of the players and staff, our thoughts go out to Liz (wife) and the family.
"Liz was just a rock for Billy over his whole life and the family gave him tremendous comfort, love, and care through his debilitating illness.
"He was the one, he was Celtic. We have lost him and it is a huge loss to the club. He was a leader of a great team and a leader of great men. I can't put into words how big a loss he is to this club."
Republic of Ireland boss Mick McCarthy echoed those sentiments and said McNeill was a huge influence on his life and career.
"It was a desperately sad day for me because he signed me at Manchester City. I had three and a half years with him there," he said.
"He had a big effect on me as a player and as a person. He didn't sign me for Celtic - Davie Hay did that, two years with him. Won the double, won the cup. He's had a big influence on my life Billy McNeill.
"I got to know all his family very well: his wife Liz, his son Martin, and his daughters - they were all there when I was with City. He was very influential in my life."