Just looked on wikipedia and it said
"..On October 30, 2008, de la Red was hospitalized after collapsing during a cup match against Real Unión. He suffered an effort syncope.[3] On December 12, the club announced the player would be missing the rest of the campaign as a precaution; and some subsequent media reports claimed that he might never recover from his heart condition.[4]
In late June 2009, after further tests proved inconclusive, de la Red has been confirmed to miss the entire 2009-10 season as well, also being scheduled for tests every two months to monitor his situation.[5] On 2 July, the club confirmed the news through its official website.[6] After new Madrid signing Raúl Albiol was given the number 18, previously worn by de la Red, the defender promised giving the shirt number back to de la Red once he eventually makes a return.."
That's a shame. I hope he gets better soon, he's a great player.