"When I think how a player should be, names like Kaka, Ibrahimovic and (Lionel) Messi come to mind," Mourinho said.
"Messi is great and in two or three years' time he will win the Golden Ball (Ballon d'Or) award."
Ronaldo hit 42 goals as Manchester United won the Barclays Premier League and Champions League last season, but Mourinho remains unconvinced that he should be labelled the greatest on the planet.
"Ronaldo is a good player but he is certainly not the best," he said.
"He deserved the Golden Ball award because his team won the Champions League and the Premier League.
"But, for me, Ibrahimovic is the best."