The Poland international says Mourinho, now in charge of Fenerbahce, was a huge support in a low point in his life.
He recalled to Il Corriere dello Sport: "It's certainly not pleasant to be booed by the fans of the club I love, but if the team's results are poor it's difficult to expect the fans to be satisfied.
"Mourinho? He was really supportive. He was very important for my career. He allowed me to play in the first team, but I could count on him not only for football: he was very close to me when my father died. Every time I have the opportunity, I will gladly thank him. I would like all coaches to have his personality."
Zalewski also revealed: "I feel like a Roma fan, I never thought about whether it would be better to leave Roma at that moment or at the end of my contract. I'm young, the most important thing for me is to play and I wanted to do it in Roma.
"The exclusion after having rejected Galatasaray? I was surprised by the club's reaction, but fortunately it's water under the bridge."