Neymar is coming to the end of an injury-hit second season at Paris Saint-Germain and the opening of the transfer window could bring fresh speculation about his long-term plans.
Madrid has been credited with an interest in the Selecao star since his departure from Barcelona in 2017 and are expected to adopt an aggressive recruitment approach under Zinedine Zidane in response to an underwhelming campaign.
Ronaldo played alongside Zidane for Los Blancos and believes his old team-mate has little chance of prising attacker Neymar away from PSG.
“It's been dragging on for many years," Ronaldo said at a promotional event for Santander.
"There are many rumours, but I don't think there's anything concrete with Real Madrid.
"He's an incredible player and anyone would like to have him but, from what I see, he's not on the market."
Madrid's rival Barcelona crashed out of the UEFA Champions League on Wednesday (AEST) as its three-goal first-leg advantage against Liverpool was overturned by a damaging 4-0 defeat at Anfield.
The collapse echoed last season's shock quarter-final exit against Roma and immediately thrust Ernesto Valverde's position into doubt.
Ronaldo defended the Blaugrana boss, suggesting the blame for such failures should be shared more equally.
"Barcelona have a great team, and they have the best player in the world in [Lionel] Messi," he said.
"The other day I heard that when they lose, it's because of Valverde, [Philippe] Coutinho... but not Messi. And when they win, Barcelona win because of Messi.
"It's a tremendous lack of respect to all the players and the coaching staff."