Reports have since suggested that Ronaldo may have chose to leave Real Madrid amid claims he is angry at his treatment by Spanish authorities and that Portugal has launched its own tax fraud probe into the world's highest paid athlete.
The 32-year-old only signed a new deal with Real in November and is under contract until 2021, though his unhappiness has caused an issue for Perez as he begins his new term as the head of the European and Spanish champions.
Madrid were on the verge of bringing former Atletico Madrid goalkeeper De Gea back to Spain in 2015 but the move was derailed as the clubs failed to close the deal before the transfer window closed.
A request for comment from Mendes' agency, Gestifute, was not immediately answered.
Mourinho was the manager Real Madrid from 2010 to 2013.
The claims against Mourinho come amid a lengthening list of tax fraud cases involving soccer stars in Spain.
Those allegations were denied in a statement by Ronaldo's management company Gestifute, while Real Madrid say they have "full confidence" in a player who "will prove his total innocence in this process".
The accusations against Ronaldo have caused speculation in Portugal and Spain that he is now considering leaving the country to play elsewhere.
He is allegedly said to owe Spanish authorities €14.7 million for unpaid taxes, also on image right, between 2011 and 2014.
A judge will decide whether there are grounds to take the matter to court.
Last year, Barcelona forward Lionel Messi and his father, Jorge Horacio Messi, were found guilty on three counts of defrauding tax authorities of €4.1 million (NZ$6.32 million) from income made from image rights.
Messi is unlikely to serve any time in prison as, under Spanish law, sentences of less than two years are usually suspended.