De Gea is almost a Real Madrid player already. Van Gaal's press conference after United's draw against Arsenal speaks for itself. De Gea is determined to join Real Madrid, but Los Blancos want to avoid a huge controversy and battle over the starting spot by allowing Casillas to leave the club gracefully. However, the Spanish captain is not willing to leave as a free agent unless he gets every penny left on his contract -- around €20 million --.
Former Real Madrid legends Raúl and Guti decided to leave Real Madrid by accepting less money than what Real Madrid owed them. They knew that their time in this club was coming to an end and signed for Schalke and Besiktas, something that Casillas will not do as easily. While Arsenal's interest in Casillas appears to be real, the British club will not pay a transfer fee for him, meaning that Casillas and Real Madrid will need to find some kind of agreement.
REPORT | Casillas still undecided about leaving Real Madrid. Club wants him to but talks between both parts have not even started.— Lucas Navarrete (@LucasNavarreteM) May 18, 2015
The talks between Casillas and Florentino Pérez should begin at some point, but both parties know that the first one to make a move might be the one losing.