Mourinho, who as sacked by Fenerbahce last week, led Roma to a Europa Conference League title triumph, their first ever trophy achieved in European competition.
"Three words to describe him? Super intelligent, skilled at his job, but I can't say the third one," Fienga told Thriving Minds.
"In private, he's very funny. It's a great challenge to be Mourinho's CEO, but it wasn't for long. In one of the first matches, we received four red cards, it was a nice welcome to Mourinho's style.
"I have a good relationship with him. Probably because I didn't work with him for very long.
"I only brought him to Roma and worked a few months together, but we're still in touch a lot."
He added: "When I decided to leave Roma, Mou came to me and said, 'Guido, there are only two alpha males in this club and now one is leaving'. I replied: 'Good for you'. That's why I've maintained a good relationship with him.
"For a CEO, working with a coach who is constantly clashing with referees and the league is like being a firefighter in California. But Mourinho is very intelligent and skilled. I have many things to say, but I can't, for both our reputations."