The former Red Devils star believes that previous boss Sir Alex Ferguson "cut corners" in his later transfer dealings and sees a big rebuilding job on David Moyes's hands.
Last year's Premier League champion currently sits seventh in its first campaign under new boss David Moyes, 18 points adrift of league leader Chelsea and even nine shy of fourth-placed Liverpool in the race for Champions League qualification.
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"United need five or six players," the Republic of Ireland assistant coach told reporters. "They've kind of cut corners in terms of transfers in the last few years, they've not gone out and got the big players and it has just caught up with them this year.
"It has maybe surprised everyone how far they've slipped behind teams but now it's a case of rebuilding. If it happens in the summer then, instead of being fearful of it, embrace it - it's exciting. These can be exciting times for United. Get them back to the very top where they belong."
The 42-year-old is adamant that Moyes needs to be given plenty of time to turn things around but is pessimistic about the club's chances of reaching the top four.
"The man [Moyes] deserves plenty of time," Keane declared. "People have been afraid to use the word that there might be 'rebuilding' going on but that's exactly the word I would use. On form, at this moment in time, you have to say the Champions League is a long way off for Manchester United.
"Would I fancy them for it? Probably not but they're still a threat, they're still there and, with their big players like Wayne Rooney and Robin van Persie back up to speed with their fitness, United will be a dangerous team if they can get their act together over the next month or two."
United faces Olympiakos in the last 16 of the Champions League.