Pires, who had a spell in Spain with Villarreal, believes Ronaldo has more improvement in him compared with Messi.
He said: "For me, there is Messi and Ronaldo, then below them are the others.
"But I love Ronaldo, I think he has more potential. People have to let him play quietly. He always has a lot on his back and that cannot be easy. He's not a machine, he's just a player."