Ronaldo made the surprise switch to Turin after nine successful years at Real Madrid for a reported €100m (£88.3m) in July.
The 33-year-old inspired his former club to four European titles in five seasons, but has yet to find the net for his new employers – albeit a three-game drought that would go unnoticed were it almost any other player.
And Messi believes that Ronaldo’s move has helped install Juventus as firm favourites for the Champions League in 2018-19.
"Obviously they (Madrid) are one of the best teams in the world, they have great players and squad, but yes the departure of Cristiano from the squad makes them less strong," Messi told Catalunya Radio.
"It makes Juventus a clear favourite to win the Champions League due to the squad they had before and now with him on top. I was surprised by his decision, I did not think he would go from Madrid nor that he would go to Juventus. There were so many teams and I heard less about Juve but it is a very good team."
Ronaldo, Messi - Getty ImagesGetty Images
Barcelona have not won the Champions League since 2015 after crashing out in the quarter-finals in each of the past three seasons.
Messi says that run must end – especially in the wake of last season’s shock defeat to Roma.
"It is necessary because we have had three years where we have gone out in the quarter-finals and the last was the worst of the lot due to how it happened," he said.
"We are concentrating on this because it is possible, we have a squad where we can fight for this competition.
"In other years the Champions League evaded us, in the Guardiola era with Inter and Chelsea, due to small details.
"Each year the competition is more even with clubs investing more money to compete and bring in players. The two Manchester clubs, Chelsea, Madrid, us, PSG and several Italian clubs."
The Argentine was snubbed from FIFA's three-man shortlist for their Best Men's Player award on Monday, with Ronaldo in the running alongside former Real team-mate Luka Modric and Liverpool’s Mohamed Salah.
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