Mourinho may have been a regular adversary in his Chelsea days but the two major footballing personalities have always maintained a healthy friendship away from the field based on mutual respect.
The Portuguese coach is in Manchester for Wednesday's Champions League tie against City at the Etihad Stadium and took time out to pay his own tribute to the United chief.
"He's unique," said Mourinho. "Especially in the modern day.
"If you go back many years, then you will find somebody like him but [it's amazing] in the modern day at the highest level, where it is really difficult to survive in our job.
"He's absolutely incredible at what he does and we can't even imagine when he'll stop. He's unbelievable."