As he has already proved this season with his five goals in four Premiership games Shearer is irreplaceable, however plans are afoot to groom his successor.
"We're always looking at how to improve our options, with or without Alan,” Sir Bobby told the Sunday Sun.
"And we're already looking for someone capable of doing his job when the time comes for him to stand down.
"There are always players around; always pebbles on the beach. But he will be a hard act to follow - there's no doubt about that.
"He is an icon, a terrific player and a legend who will always be remembered here. So yes, he will be hard to replace.
"But not impossible."
But Sir Bobby doesn’t believe that he will be succeeded by Shearer as United manager.
He added: "I think he will go into the media, to be honest. I don't think that, as a multi-millionaire many times over, he needs the hassle of management.
"Alan certainly has several of the qualities required of a top manager. He is well respected, he has a personality, a presence about him and he isn't afraid to speak his mind.
"But maybe he doesn't want to go on the pitch and shout and holler at players.
"He may feel he doesn't need to work. He's made a lot of money, he's done well on TV and could get a nice contract from Sky.
"Plus, these days it is mandatory to have a UEFA pro coaching licence to be a Premiership manager, and he hasn't got that."