Paddy Crerand:
"Cantona will top many polls about Fergie's best ever buy but for me Roy Keane was the No 1.
"Eric was brilliant but it was always Roy for me who was the driving force behind all that success. You need a real leader and Keano was just that. I don't think United would have won as much as they did without Roy in the side.
"He deserves to go down in the Old Trafford history books as one of United's greatest players. He had the will and unbelievable desire to win. He was the manager in a football kit. How often does that happen when a player mirrors everything about the manager?
"All footballers want to be winners but Roy had that extra edge that put him beyond most others in the hunger stakes just like Fergie as a boss.
"Bryan Robson was exactly the same. If Sir Alex was able to go out and buy a Keane and a Robson this summer then you might as well say next season is over. You could wrap it up, say `thank you very much' and hand United the Premiership title, the Champions League, FA Cup and Carling Cup!
"That's why it has been and will be so tough and probably more or less impossible to replace Keane."
Billy McNeill (ex Celtic captain and manager):
"I hate to say it but I warned Gordon Strachan against signing Roy Keane in the first place and yesterday my fears were realised. Celtic were never going to get sort of return from the veteran midfielder that they were looking for. Just 13 games and one goal in six months in Scotland - if I'd still been in charge then I would have been bitterly disappointed with that. But then if I had still been in charge at Parkhead I wouldn't have signed him. Yes, he was once arguably the best midfielder in the world but Celtic were always getting a player who was trudging towards the end of his career. Because of the punishment his body had taken in over 10 years at the top in England he was never going to set the heather on fire in Scotland. The glory days were firmly behind him. I knew it, he knew it and more worryingly for me, Celtic knew it. For me the whole exercise has been a gamble that hasn't paid off."
Stilian Petrov (Celtic):
"I feel that it is really disappointing for football, not just Celtic. It was a great honour for me to play with Roy. Every game with him was special and I feel very lucky because not too many players have had the chance. I am really pleased that I had the opportunity and I am sure every other Celtic player feels the same. Roy achieved so much as a footballer and a person – and I am very sad he has played his last game."