The Spanish team said on its website that Diarra is "resting at home" and will have treatment for "asthenic syndrome secondary to a sickle-cell anemia."
Diarra had missed three days of training in the French Alpine resort of Tignes. His illness was apparently triggered when the team slept at an altitude of around 9,800 feet Wednesday night.
Coach Raymond Domenech said Diarra "had no problems" when he trained at the same resort two years ago before the European Championship.
Domenech has yet to decide whether to call up a replacement ahead of Wednesday's home exhibition against Costa Rica.