Sharpe, 41, reckons that for as long as United stay in the frame for major honours, he will keep the hunger to prolong his dynasty.
"Sir Alex is unique and with him you feel anything is possible," he told the Daily Star Sunday.
"He has said in the past that he will retire but I can see him going on and on.
"As long he has the desire and good health then why not? I don't know how long he can go on but they may have to ask him gently to call it a day when he gets to 80!
"But seriously, pundits are debating who will be next in line to be manager at Old Trafford and I think they are premature to do so. It'd be a brave man who has the task of telling him it is time to retire.
"He is remarkable and the list of honours he has with United is testimony to that."