Ronaldo, 33, made a surprise move from Madrid to Juventus for a reported €100million in July after spending nine seasons with the LaLiga giants.
Madrid initially struggled to cope with Ronaldo's absence and are eight points adrift of league leaders and rivals Barca - although they hold a game in hand.
Messi is contracted at Barca until 2021, but the club's former midfielder Cruyff urged them to learn from Madrid's mistakes.
"What happened to Madrid is a warning for Barca," the Chongqing Dangdai Lifan coach told Sport.
"Madrid players are still great, but in football you need that player who even losing 2-0 you think you can win. It happens with Messi.
"It's much more than football... it is belief."
Messi has enjoyed another stellar start to this season, scoring 21 goals in just 20 games in all competitions.
Cruyff, a former Netherlands international, said Ronaldo – like Messi at Barca – was holding Madrid together.
"You analyse Madrid and you see that Cristiano was the man who holds the umbrella," he said.
"When it rains, nobody gets wet. Now, there is no umbrella."