Louis Van Gaal wants to sell David De Gea this summer after the goalkeeper's mediocre performance against PSG, where he appeared to be unsettled. Van Gaal said after the game that this situation "is not good for Manchester United, De Gea himself or the club interested in him," which means that De Gea's departure might happen soon.
Real Madrid will reportedly buy the goalkeeper for €35 million, and United will then spend some of that on Ajax goalkeeper Cillessen. The Red Devils have also signed Sergio Romero as a free agent, so they have De Gea's two potential replacements ready to come in and fight for the starting spot.
If De Gea joins Real Madrid this summer Keylor Navas will most likely leave the club. The Costa Rican wants to play and that would not be possible with De Gea on the club. Kiko Casilla would be Real Madrid's reserve goalkeeper.