Ronaldo, who played under Mourinho at Real Madrid, has signed a four-year contract with the Italian champions, who have won the last seven Scudetto and are trying to transfer their domestic dominance to the European scene.
Mourinho has been vocal in his praise of the signing, but he seemed uncertain as to whether the it would cement Juve's dominance or inspire others to rival them.
"It is fantastic for Italian football: the Bianconeri will dominate for years and now it will be even easier with a player of [Ronaldo's] qualities," Mourinho told ilBiancoNero. "But, I think it can be extra motivation for their rivals. Maybe this supremacy of Juventus can give AC Milan, AS Roma, Inter Milan and Napoli a boost in the title race."
Juventus were given a run for their money last season by Napoli, who led the title race for several months before eventually falling back and finishing four points behind Max Allegri's team.
Mourinho believes that the signing of Ronaldo will reinvigorate Serie A and put it on a par with La Liga and the Premier League as one of the biggest leagues in Europe.
"La Liga no longer has the two best players in the world. Now one is in Spain and the other is in Italy," the Manchester United boss said, quoted by the Mirror. "Now everyone looks at Italy for Ronaldo, the Premier League because it's the best league and Spain for [Lionel] Messi.
"Serie A has become one of the most important championships in the world. In football, everything can change."