Messi and Ronaldo have contested one of the great sporting rivalries over the course of the last decade, competing for individual and team honors while playing for archrivals Barcelona and Madrid, Sky Sports reported.
However, the pair will now only meet on the field again if Madrid draws Juve in European competition, following Ronaldo's £105m move to Turin.
Messi said Real is now worse off without Ronaldo and believes the Spanish club's loss is Juventus' gain.
"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 favorite 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."
Barcelona has not won the Champions League since 2015 – largely due to the dominance of Real Madrid, which has claimed the trophy in the last three seasons.
Messi said Barcelona is desperate for European success this year after three consecutive quarterfinal exits.
"It is necessary because we have had three years where we have gone out in the quarterfinals and the last was the worst of the lot due to how it happened," said Messi.
"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."