The Irishman's contract is due to expire in the summer and it was widely assumed he would be given at least an extra year, possibly to combine playing duties with a coaching role.
However, in a live MUTV phone-in, Keane has rejected any likelihood of moving straight on to Sir Alex Ferguson's backroom staff and feels it will be the end of the season before United chief executive David Gill decides whether the 34-year-old will be handed an extension, by which time it will be too late.
"I would like to play another year or two but I do not think it will be at Manchester United," he said.
"I would be surprised if I was offered a new contract and even if I was, I would not expect it to be until the end of the season but by then I will have already made a decision about what I am going to do.
"There comes a time for everybody when they have to move on and I am prepared to play elsewhere.
"I think it will be good to experience a different dressing room. It wouldn't be an English team though, coming back to Old Trafford and going into the away dressing room would be too hard for me to stomach."
The last statement is bound to alert Celtic, as Keane has made no secret of his desire to play for the Bhoys at some stage in his career.
Although he is currently sidelined with a broken foot, the prospect of United without Keane is virtually unthinkable.
The fiery midfielder has been the inspiration behind Sir Alex Ferguson's side since he first arrived as a raw youngster from Nottingham Forest in 1993, making 480 senior appearances and playing a major role in almost all United's successes.
Even though he missed the 1999 Champions League final, the £3.75m Ferguson splashed out to secure his services will probably go down as some of the best money ever spent by United.
However, it appears the United boss is losing his battle to keep Keane at the club.
When the Cork-born player first mooted his intention to quit this summer in the middle of last season, he immediately received a telephone call from Ferguson informing him the Scot would be the man who decides when the midfielder will sever his ties with Old Trafford.
Ferguson's position is not likely to have changed. Indeed, he will probably be reeling at this fresh announcement.
But the harsh reality is if Keane is offered a new deal, it would almost certainly be for one year only and then on a pay-per-play basis, whereas Celtic would almost certainly offer a better overall deal.
"I am not putting a gun to anybody's head," said Keane. "But my gut reaction last season was this would be my last year and I still feel that way.
"You have to learn and be prepared to move on. Life will not stop when I leave Manchester United.
"It might be an opportunity to go into management or coaching somewhere else and it is best to make a clean break because coaching at Manchester United doesn't really appeal."
Keane has made no secret of his desire to follow former Red Devils skippers Steve Bruce and Bryan Robson into management and intends to complete his Uefa coaching badges over the next few months.
But, interested lower league clubs who feel they may be able to attract the most influential domestic player of his generation, might be advised to look elsewhere.
"I don't want to look too far ahead because I want to play on for a bit yet but I wouldn't want to drop too far down," he said.
"Your first job in management is an important one. I would like to go to a club with ambition, a decent fan base and some decent players."
