MADRID (AFP) - Real Madrid said Arjen Robben will be unavailable for Thursday's Spanish Cup match against Mallorca due to a muscle strain, the latest injury to the club's costliest pre-season signing.

The Dutch midfielder has now suffered four injuries and failed to make an impression since joining the club from Chelsea for 36 million euros (52 million US dollars) on a five-year contract.

The 23-year-old has made only five starts since joining Real Madrid, two of them in the league, the newspaper El Mundo noted on its website.

It said Robben, who gets paid around 10 million euros a year, has thus lived up to his reputation of being as fragile "as glass".

El Mundo said his fraility contrasted with the robustness of countryman and clubmate Ruud Van Nistelrooy, 31, who on Wednesday extended his contract for two years to 2010.

Coach Bernd Schuster said last month that Robben was suffering from "anxiety" resulting from his desire to prove himself.