Carlos spent three seasons with the Blaugrana between 1991 and 1994, winning three league titles and a European Cup in an extremely successful spell at Camp Nou, which accompanied two stints at Real Vallodolid and stops at Atletico Madrid and Valencia.
Speaking in an interview with Sport.es, Juan Carlos, now the Valladolid technical director, was not shy about sharing his opinions on Mourinho's impact at Los Blancos.
"With Mourinho's tenure, in terms of his performance as a coach, we have to look at hard numbers," Juan Carlos said. "And if we do that then the balance has not been good. But then again, in terms of other aspects, the balance has been far worse.
"The image of Real Madrid as a club has been badly damaged, regardless of whether he was right or not with some of the things he said. Elsewhere at Chelsea, Inter and so on he made changes and things went well.
"But here he has done more harm than good. And he has not achieved the success he pursued."
Mourinho has been strongly linked with the Madrid exit door this summer and a return to former club Chelsea, with Paris Saint-Germain boss Carlo Ancelotti tipped to be his potential successor at the Santiago Bernabeu.