Ronaldo joined Juventus for £88 million after winning three consecutive Champions League titles with Zinedine Zidane.
Real lost their Super Cup game against Atletico Madrid, but have won three games in a row to start their LaLiga campaign.
They have a new coach, Julen Lopetegui, who left the Spanish national team on the eve of the World Cup after news broke of his appointment.
Ronaldo et Messi lors des trophées The BestGetty Images
Speaking to Catalunya Radio, Messi said Real were now weaker due to Ronaldo's exit, and that Juventus had improved as a result:
"Real Madrid are one of the best teams in the world and they have a great squad [but] it's evident that the absence of Ronaldo makes them less good and makes Juventus [one of the] clear favourites to win the Champions League," Messi told Catalunya Radio.
"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 has gone to a very good team."
Messi said it was now the time to win the Champions League again with Barcelona, having won it four times with the club. Their last success came in 2015.
"It's time. We have been knocked out in the quarterfinals in the last three seasons. Last season's result in Rome hurt especially, but we have a spectacular squad and we can fight for it."
This article was written by cool news network.