Celtic tried "desperately" to keep Aberdeen-bound skipper Scott Brown at the club beyond the end of the season, chief executive Peter Lawwell has declared.
Brown is following the retiring Lawwell out of Celtic Park this summer after playing for the club for 14 years, more than 11 of them as skipper.
The 35-year-old's impending departure adds to the sense of transition, with the club looking for a replacement for Neil Lennon – some bookmakers have suspended betting on the new manager market following a flurry of gambles on Roy Keane.
Brown has won 22 trophies for Celtic and will aim to bow out with another Scottish Cup triumph.
Lawwell said on the club's website: "Scott Brown has been the heartbeat of Celtic for so long and without doubt, the most influential figure in the Scottish game across the past 15 years.
"With more than 600 appearances for the club, he has made an imprint on Celtic which puts him in the company of our other true greats – and with a legacy to match.
"Ever since signing, he has lived and breathed the club and our cause – day after day, across a decade-and-a-half, giving his heart and soul to Celtic.
"As every Celtic manager in the past 15 years will testify, as well as being a hugely talented player, Scott's effort, his drive, his work ethic and his will to win, are an example to us all.
"He has been a pivotal figure in delivering such a sustained period of dominance for the club and for that, I know every Celtic supporter will be so grateful to him.
"Scott has had hugely difficult times in his life away from the pitch and his resilience and determination to face any challenge has been quite extraordinary.
"He is a tremendous man, someone of real quality and integrity and someone who I personally have been honoured to have at my side as Celtic captain.
"It goes without saying that we were desperately trying to keep Scott at the club, but while of course we are very saddened by his decision, we understand and respect it fully and wish Scott and his family nothing but success, health and happiness for the future."
Interim manager John Kennedy admitted it would be impossible to find someone else in the same vein.
"He's been a terrific captain and leader of the group," Kennedy said. "He'll be irreplaceable in terms of what he gives us on a daily basis, his drive to be successful and how he leads the group by example every single day.
"He's the perfect example of a modern-day professional. How he lives his life, the condition he keeps himself in and how he pushes himself every single day is incredible.
"He's the guy coaches always look to in order to take some of the burden off other players. You always know Scott's there and every matchday you hear him before games and you hear him on the pitch.
"He drives the training forward every day. He's a massive character, a huge personality and he's been a massive part of this club for so long and he'll certainly be missed.
"I'd love to have had more time with him and seen him bring more success to the club but it's his own personal decision and we have to respect that. He's given so much of his life to us and we wish him all the best moving forward."
