Gareth Bale suffered "fatigue in his left thigh muscle" when an injury in the warm-up forced him to pull out of Real Madrid’s 4-1 win over Getafe at the weekend, the club have said.
Bale, the world's most expensive footballer, had been due to make his home debut in that match. He now appears certain to play no part in Wednesday’s trip to Elche.
A statement on Madrid's official website said: "After the tests carried out today on Gareth Bale at the Sanitas La Moraleja hospital, fatigue in the left thigh muscle was found. It will be treated."
On Sunday, Isco replaced the Wales international in the starting line-up and went on to score, with Raphael Varane filling in on the bench.
Speaking about Bale after the match, coach Carlo Ancelotti said: "He had a small problem when he was warming up.
"It's not an injury -- he just didn't feel comfortable. He is disappointed, because he was very excited to play. I don't know if Bale can play on Wednesday because it's difficult for a player to be ready in three days."
However, the coach said he expected Bale to be back in the squad for the weekend's derby against Atletico Madrid at the Bernabeu.
"I think Bale will be ready for Saturday's match," he said. "We are working to improve Bale's fitness because he didn't have a good pre-season."