"Having received medical advice from my surgeon and the Celtic club doctor, I feel my only option is to retire," said Keane in a statement. "I would like to send the manager, the staff, the players and the supporters of Celtic Football Club my best wishes for the future."
Among the best midfielders of his generation, Keane started a glittering career with Cobh Ramblers before being snapped up by Nottingham Forest in 1989. Keane quickly made an impression at The City Ground and was renowned for his all-action style of play. He joined United in 1993 for £3.75 million after shunning Blackburn Rovers and he went on to win a string of major honours after proving to be the heartbeat of Sir Alex Ferguson's team.
But suspension ruled Keane out of the club's finest hour of modern times when he missed the 1999 UEFA Champions League final against Bayern Munich after a heroic performance against Juventus in the semi-finals. Following his surprising split from United last autumn, Keane agreed a deal with Celtic and featured in enough games to secure a Scottish title medal.
He had an option for a contract for next season, but Keane has decided that his injury worries are serious enough to prevent him from helping Celtic in the Champions League. Keane's career has also not been without controversy as he famously left Ireland's 2002 World Cup squad in the Far East after a bust-up with Mick McCarthy.
The hothead midfielder was also regularly in trouble with referees and was punished after the event by The Football Association when he admitted to deliberately going out to injure ex-Manchester City player Alf Inge Haaland.
I have to say although I am a Man City fan, he was a brilliant player although he could be a ploncker at times with some of the tackles he made. I wish Roy and his family the best of luck in the future and he will always be remeber as a "LEGEND" by his fellow country men.