Ibrahimovic played under Mourinho for a year at Inter Milan before joining Barcelona in the summer of 2009. After winning the treble of Serie A, Coppa Italia and Champions League titles, Mourinho has also moved to Spain - to coach Barca's fierce rivals Madrid.
"It's a big challenge for Mourinho," Ibrahimovic said at a press conference in Oslo, where Barca are currently on tour on pre-season.
And he added: "He is the best or one of the best coaches in the world. I don't believe in Madrid, but I believe in Mourinho."
Ibrahimovic himself has been linked with a move to Real, but says he is happy at Barca.
"I'm at the best club in the world," he said.
"Last year in Italy I had won just about everything. I needed big challenges and that's why I took the step (to join Barcelona)."