Jan Oblak
Jan Oblak is on top of United\'s list of players to succeed David de Gea Reuters

Manchester United have stepped up their interest in Atletico Madrid goalkeeper Jan Oblak after Iker Casillas' move to Porto was made official.

The Sunday People reports that the goalkeeper is on top of United's list of players to succeed David de Gea after his potential move to Real Madrid in the next few days. The Red Devils have already signed three players thus far, starting with Memphis Depay from PSV Eindoven and followed quickly by the transfer of Bastian Schweinsteiger from Bayern Munich and Matteo Darmian from Torino.

De Gea was among United's best players last year, making his way into the PFA Team of the Year and should the move to Madrid go through, he will be a massive blow to United's chances of challenging for the title next season.

Meanwhile, Atletico are demanding £30m ($47m) to part ways with Oblak, but the report states that anything in the range of £21.5m ($33m) should be enough for United to sign the player. Van Gaal rates the 'keeper very highly and keeps him in the same bracket as De Gea, who has slowly but surely established himself as among the world's best stoppers.

They already have former Barcelona keeper Victor Valdes as back-up, but the Spaniard is reportedly being targeted by Turkish outfit Antalyaspor, who are prepared to take him off their payroll this window. Valdes joined United in January and made only two appearances for the Manchester based club, owing to an injury to De Gea in their game against Arsenal.

The Red Devils have also been linked with Jasper Cillessen, with the Mirror reporting that the Dutchman is on top of United's list of players for the summer. The Ajax keeper is on very good terms with Van Gaal, having worked with him in the World Cup, but the Eredivisie club are unwilling to let him leave this summer.