The clubs agreed a £29.5m deal for De Gea on the final day of the summer transfer window but it broke down at the last minute after the relevant documentation was received in Spain 28 minutes after the deadline.
That sequence of events led to both clubs to issue statements and in United’s, they said they offered Real Madrid the chance to make an appeal – but Perez has doubts about their intentions.
“They said that just to get in good (with the supporters). It is just to say we offered them that but Real Madrid rejected it,” he said.
“But the paperwork arrived later. It was out of time. We sent to La Liga anyway, we spoke with the president and with a chief from UEFA but we saw it was impossible. Their offer (to try to help us to get the deal done) was from someone without experience.
“I think they didn’t have bad intentions. I think they just lacked experience. It already happened to us (two seasons ago) with Fabio Coentrao and also happened to Athletic Bilbao with Ander Herrera (the summer before the Spanish midfielder made the move when David Moyes was in charge).
“When this happens more than once and you see that all the operations that, according to the media, they have tried to do during the summer (which didn’t go through), one believes that this happened because it is a new team with no experience.”
Perez was also strongly critical of the delays in United submitting the relevant documentation over the transfer and said: “It was a disgrace. I do not want to blame anyone, but we do not understand why they took eight hours to check the contracts or that they didn’t open negotiations until 12 hours before the deadline.”
He added: “In half an hour, we agreed everything. We sent them the paperwork at 1.30pm and it took eight hours for them to return it. I do not think they did it with malice.
“They probably would be very busy but one thing that you can do in an hour takes eight (hours) for them. We, Real Madrid and FIFA have proof but we do not want to make it public.”
United hit back by suggesting that Perez had made such claims to mask his club’s own failings.
A spokesman said: “The facts speak for themselves. The FA are prepared to back our case that documents were in on time; Real seem intent to move the focus away from their own clumsiness this summer.
“We all like to blame others but if you let one slip through your fingers into the back of the net, then ultimately the culpability is yours.”