The formal announcement from Old Trafford was greeted with amazement throughout the football world.
The formal announcement from Old Trafford was greeted with amazement throughout the football world, although it came less than three weeks after Keane delivered an attack on his team-mates, deemed not-fit-for broadcast on MUTV.
At the time, it appeared the editorial decision - instigated by Alex Ferguson and chief executive David Gill - might only serve to fuel Keane's resentment with the falling standards at United.
Instead, the Corkman's assault punctured a hole in his relationship with Ferguson which could not be healed.
In launching personal attacks on his team-mates, Keane crossed the line which Ferguson has made one of the central components of his entire managerial career.
"You don't criticise any Manchester United player outside the doors. I have never done and I won't," said Ferguson prior to beating Chelsea on November 6, 2005.
"I am unremitting in that respect. Totally unequivocal. My stance is there and it doesn't change."
Ferguson did not name his captain outright, but there was little doubt where his ire was aimed and sources close to the United boss claimed he was "fuming" at Keane's volley and was no longer prepared to stand behind the man regarded as the talisman of his time in charge.
So, after a turbulent few months which saw Ferguson and Keane at loggerheads over United's pre-season training camp in Portugal, before the Irish skipper then used a live MUTV phone-in to declare this season would be his last with the Red Devils, the United hierarchy decided to sever ties completely.
"It has been a great honour and privilege for me to play for Manchester United for over 12 years," said Keane, who broke his foot in what turned out to be the last of his 480 appearances for the Red Devils against Liverpool in the September.
"During my time at the club, I have been fortunate to play alongside some of the best players in the game and in front of the best supporters in the world.
"At all times, I have endeavoured to do my best for the management and the team.
"Whilst it is a sad day for me to leave such a great club and manager, I believe that the time has now come for me to move on. After so many years, I will miss everyone at the club."
Seven English titles, four FA Cups and the European Cup - the final of which he was suspended from - offer their own epitaph on Keane's time at Old Trafford, and even at the end, neither Ferguson nor Gill were prepared to diminish his contribution.
"Roy Keane has been a fantastic servant for Manchester United," said Ferguson.
"The best midfield player in the world of his generation, he is already one of the great figures in our club's illustrious history.
"Roy has been central to the success of this club over the last 12 years and everyone at Old Trafford wishes him well for the future."
Gill added: "Roy has been a towering figure at this club for over a decade.
"His dedication, talent and leadership have been qualities that have marked him out as one of the true greats."
Keane joined Celtic before managing Sunderland and Ipswich. He was also the assistant manager for the Republic of Ireland, and now works as pundit for ITV and Sky Sports.