The former Real Madrid captain completed a move to Portuguese side Porto last week after the goalkeeping legend was deemed surplus to requirements by manager Rafael Benitez.
Portugal international Ronaldo, meanwhile, sold his image rights to Valencia owner Peter Lim in a reported £35m deal earlier this month.
The 30-year-old forward has been linked with a return to Manchester United regularly over the past 24 months amid reports of numerous fallouts with the Madrid hierarchy.
And Casillas’s agent, Santos Marquez, claimed that Ronaldo’s decision to sell his image rights to the Valencia owner in a six-year deal suggested that the forward could be set to leave The Bernabeu.
“One day [Perez] will need to explain why Cristiano works for another company outside of Real Madrid,” Marquez told AS.
“Imagine Cristiano generating €40 million elsewhere and giving it to Peter Lim, Valencia’s owner. It ends up in Valencia’s coffers and that’s deplorable.
“Will anyone speak out and ask Florentino why Cristiano has sold his image rights to Peter Lim? That’s not normal; it suggests that Cristiano is planning to leave.”
Ronaldo moved to the 10-time European champions from the Red Devils in a then world-record £80m deal in 2009.
The former United winger has won one La Liga titles, the Copa del Rey and Champions League during his time at Madrid.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news