Messi and Ronaldo battled against each other in La Liga for nine years with their respective sides but the Portuguese forward left for Serie A in July.
Real Madrid and Barcelona have won their opening three matches and are joint top of the table on nine points.
"Real Madrid are one of the best teams in the world with a great squad (but) it's evident that the absence of Cristiano Ronaldo makes them less good and makes Juventus (one of the) clear favourites to win the Champions League," Messi told Catalunya Radio in a show broadcast on Monday.
"It surprised me. I didn't imagine him leaving Madrid or joining Juventus. There were a lot of teams that wanted him. It surprised me but he went to a very good team."
In early August Messi gave a speech in which he said Barca wanted to win the Champions League this year after the disappointment of being knocked out by AS Roma in the quarter-finals last season.
The No. 10 reaffirmed his desire to win the trophy Barcelona last lifted in 2015.
"It's time. We have had three years in a row being knocked out of the Champions League and are hurt further still from last season's result in Rome," added Messi.
"We have a spectacular squad and we can fight for it. We lost chances to win it in the (Pep) Guardiola era against Inter (Milan) and Chelsea. We were better than them and we could not get to the final because of small details."
Barcelona face Inter, Tottenham Hotspur and PSV Eindhoveni the group stage of this season's competition.
(Reporting by Rik Sharma; editing by Ken Ferris)
