Valdano - along with a number of other Real Madrid employees - famously did not quite see eye-to-eye with Mourinho during the latter's spell in charge at the Santiago Bernabeu.
The pair supposedly first fell out over a request to sign a new striker as cover for then Los Blancos frontman Gonzalo Higuain and the relationship seemingly never recovered.
The former Argentine eventually left Real having been sacked in May 2011, but has pulled no punches on the subject of the fiery Mourinho in his new book.
"I've never heard in public or in private a football statement from Mourinho worthy to be remembered," he was quoted by the Spanish press in excerpts that were brought to our attention by the Daily Express.
"I couldn't understand him because he is in the antipodes of my sensitivity. Intelligence and ego are enemies. And when they collide, the ego wins."
Valdano was also keen to stress his belief that Barcelona favourite and current Bayern Munich manager Pep Guardiola - Mourinho's great rival during his time in Spain - is superior.
"If Guardiola is Mozart, Mourinho is [Italian composer Antonio] Salieri. He would be a great musician if there were no Mozart," he added.
Guardiola v Mourinho
Given their respective successes and indeed their hugely contrasting personas, the comparison between Guardiola and Mourinho is one that has been made frequently over recent years.
In terms of reputation, there is little to choose between the two. Mourinho has the upper-hand when it comes to general experience, while both can lay claim to two Champions League title successes.
The Portuguese guided an unfancied Porto side to victory over Ligue 1 outfit in Monaco in 2004 before repeating the feat six years later with Inter Milan.
Both of Guardiola's successes came during his historic reign in charge at Barcelona and it is expected that he will soon steer his formidable Bayern Munich outfit back to the very top.
Who do you think is the better manager - Guardiola or Mourinho? Why? Are Valdano's comments fair? Let us know your thoughts on the subject in the comment box provided below.