Calleja recently left Al Riyadh and recalled facing Ronaldo when playing against Al Nassr in the Saudi Pro League.
He told Marca: "I think our opinion of Cristiano Ronaldo is very similar to that of any fan watching from home. He's a special footballer, undoubtedly one of the best in history. In terms of competitiveness, professionalism, and ambition to always be the best, he's possibly number one.
"Anything good said about him falls short, because he's an example of how to work tirelessly to achieve excellence. He's a different kind of player, special, and, in a way, touched by a magic wand.
"I think Cristiano Ronaldo's great secret is how he works on his physique, his mentality, and everything surrounding his football, which makes it practically timeless. He's a player who has achieved everything he has through hard work and being the most professional in every detail.
"He takes that level of commitment to the extreme, and that's reflected in the level he maintains at an age when others could no longer compete like that. Anyone who didn't take care of themselves in that way would be incapable of performing as he does now. That's why, more than a question of age, it seems like he's going to play as long as he wants."