Jorge Mendes said the former Manchester United forward was the “best sportsman ever” and a unique talent.
“If for any reason the club decide to sell him for Â£300m, someone will pay,” Mendes told sports editor Dan Roan.
Ronaldo, 29, was voted the world’s best player for a second time last month, beating Lionel Messi to the award.
However, Mendes insisted his countryman, who cost Real Madrid a world record fee of Â£80m in 2009, would stay at the Bernabeu, despite his “love” for Manchester United supporters.
Mendes, who is reported to have brokered more than Â£1bn worth of transfers, has been described as the most powerful figure in the game.
He described football as the “most important sport in the world” and defended players’ wages, saying they “deserve the maximum possible”.
Mendes, a former DJ and nightclub owner, also represents Jose Mourinho, Luiz Felipe Scolari, Radamel Falcao, Angel Di Maria, James Rodriguez, David De Gea, Victor Valdes and Diego Costa.
Source : BBC Sport