Ahead of today's FA Cup tie, Clough smiled: "It's hard for me to say because of my family connections - if I said Sir Alex, a bright light might come crashing down from above and hit me on the head!
"He is certainly up there. The length of time he has been doing it is the most staggering thing in modern football.
"In some ways I suppose it's easier being at Manchester United, the biggest club in the world, but in other ways it's harder because you have to maintain those standards.
"A different team evolves every few years because you have youngsters coming through mixed in with big-money signings. He has never been afraid to spend money, and when you are at the biggest club in the world, why not?
"You can't really compare his achievements at United with my dad - in Sir Alex's case, I suppose winning a European trophy with a small, unfashionable club like Aberdeen stands better comparison."