Law believes Ferguson, who turns 68 on January 31, will retire when he is 95 or 100.
“When Alex retires maybe at 95 or 100, he will probably turn out to be the greatest manager the game has ever seen,” Law told MUTV’S Red Cafe programme.
“When you look at the great managers, Busby, Shankly and Stein, he will be in that category.
“They all came from working-class backgrounds and football has always been a working-class game.
“There is a passion among Scottish managers that other countries do not have.
“Coming from a hard-working area they wanted to entertain the people but those days have gradually gone.”
Law added: “You go back to his time at Aberdeen, the main teams in Scotland are Celtic and Rangers. He came in and knocked them down. The success he had was fantastic.
“After he came to United things didn’t go too well and if they hadn’t won the FA Cup in that particular year, there might have been a change.
“But United realised you can’t come into a club and have instant success and stuck by him.”
