De Gea almost moved to Real five years ago before the deal collapsed as the paperwork was not lodged in time due to a fax problem.
Mendes was still tasked with sorting the 29-year-old's £375,000-a-week United extension in September 2019.
But the pair's relations have reportedly been severely damaged by a dispute during the final stage of the 18-month negotiations.
The lengthy talks allegedly saw De Gea become 'frustrated' with Mendes.
The eventual contract then had the Portuguese ignored on the final FA paperwork in favour of the player's family lawyer, Jose Bouzas Aragon.