The accusation against Mourinho comes a week after the Madrid-based prosecutor's office accused Real Madrid forward Cristiano Ronaldo of defrauding Spain's tax office of 14.7 million euros ($16.5 million).
"I will always defend Cristiano as a person and as a footballer".
The Spanish prosecutor's office announced on Tuesday that it has filed a complaint in a Madrid court against the Portuguese coach regarding tax irregularities.
And finally, they accuse him of "voluntarily" refusing to include 28.4 million euros in income linked to the sale of his image rights for the 2015 to 2020 period to a Spanish company.
Ronaldo is now on global duty with Portugal at the Confederations Cup, but, according to the report, has told his teammates that Perez's words were "better late than never" and have conveniently led to him having a change of heart.
There has been speculation that Ronaldo is trying to put pressure on Real so that they will bear some of the burden of an eventual fine as Barcelona did for Lionel Messi.
And Perez would not be drawn on a summer move for United goalkeeper David de Gea, for whom a transfer fell through two years ago, potentially as part of a deal that would take Ronaldo, Morata, or both, to Old Trafford.
"As a rule, we do not comment on these rumours".
'Nobody wants him to go, ' said Perez.
Victory meant Real became the first team to win back-to-back Champions League titles, with Ronaldo the competition's record goalscorer with 105.