Real Madrid coach Rafael Benitez fears Welsh star Gareth Bale could be sidelined indefinitely due to a calf injury.

Bale was forced off after just half an hour of Madrid's 4-0 win over Shakhtar Donetsk to open the Spanish giant's Champions League campaign on Tuesday.

Karim Benzema opened the scoring before Cristiano Ronaldo struck his second hat-trick in as many games to move clear of Lionel Messi as the competition's all-time leading scorer with 80.

"Great win at home to kick off our #UCL campaign, gutted to go off but hopefully nothing too serious," Bale posted on his official Twitter account.

However, Benitez was more pessimistic over his chances of facing Granada on Saturday.

"Bale had a spasm in his calf muscle," said Benitez.

"We must wait a day or two to see how serious it is. He had a similar issue last year."

Bale was sidelined for nearly three weeks with a calf problem last season that forced him to miss the second leg of Real's Champions League quarterfinal against Atletico Madrid.

Should the injury be more serious, he could also miss out on sealing Wales' qualification for an international tournament for the first time since 1958 when it travels to Bosnia and Herzegovina before hosting Andorra next month.

Defenders Raphael Varane and Sergio Ramos also had to be replaced due to minor ankle and shoulder injuries respectively, but Benitez is more confident they could be fit for the weekend.

"Obviously we have to wait, I said before two of them are knocks. Gareth is the only one that I am a little bit more negative with but I have to wait to see what the doctor says."

Early season fears that Benitez's Madrid might not have the same attacking prowess as in previous seasons have been dispelled as Los Blancos have now scored 15 times in their past three outings.

However, the former Liverpool and Chelsea boss was also keen to stress the importance of keeping its fourth consecutive clean sheet against a talented Shakhtar side.