Capello: Man Utd boss Ferguson greatest ever
Capello and Ferguson clashed when the Italian was in charge of England, but have been long-term friends away from the game.
"He was the greatest coach of all time, especially because of all the difficulties he overcame at the start of his career with United," Capello said.
"I normally think, after four or five years at the most, a coach should move, so to think that he stayed on the same bench for 26 years is incredible. That takes psychological, technical and humane capacity."