Celtic chief executive Peter Lawwell admitted the Hoops were desperate to keep hold of Scott Brown.
The former Scotland international will join Aberdeen in the summer after agreeing a two-year deal to become Stephen Glass’ player-coach.
It brings an end to Brown’s trophy-laden spell at Parkhead where he guided the Hoops to 10 league titles and 12 domestic cup and made more than 600 appearances.
Lawwell said: “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.
“When he chose Celtic over others, we were of course delighted.
“We knew we were getting one of the country’s finest talents and he in turn a chance to take to the stage and be part of Scotland’s biggest club.
“He took this opportunity with both hands but I don’t think anyone of us would have imagined just how successful he was going to be.
“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.”
He added: “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.
“We know Scott will continue to give everything to the club until the end of the season and as he always does will be working tirelessly to bring the club and our fans further success.
“I know our fans will miss Scott in a Celtic jersey and fighting for our badge, something he has done for so long. However, there is no doubt he will forever be remembered at Celtic as our captain, leader and indeed a legend.”