Bale was substituted at half-time in last weekend's 2-0 win over Granada due to the problem and has been unable to train with his teammates all week.

The 24-year-old has now missed a total of eight games to injury in four separate spells since joining from Tottenham Hotspur for a reported 91 million euros ($122.7million) back in September.

However, Madrid boss Carlo Ancelotti is confident he will be fit in time for the first leg of Real's Copa del Rey semi-final against Atletico Madrid on Wednesday.

"He is not available for tomorrow because he hasn't trained all week with the group," Ancelotti told a press conference on Saturday.

"Tomorrow he will train and he will be available for Wednesday. He doesn't need another mini pre-season. The problem he had was very small, he just needs to train and gain confidence.

"His problems don't worry me. Before he had a knock and was out for 20 days, but when he came back he played well. Now he has a small problem, but it is not a serious injury."

Madrid enjoyed a perfect January as they racked up eight wins and eight clean sheets to move to within a point of Atletico Madrid and Barcelona at the top of La Liga, as well as progressing to the last four of the Cup.

However, Ancelotti knows that facing an in-form Athletic team, who have scored 15 goals in their last three league games, will be a stiff test of his side's title credentials.

"It is a very good test looking at the table. They are a very strong team and they don't need to prove anything.

"We have a period of important games coming up. January was a good month and we have the ability to repeat what we did in them in February. What we did in January could allow us to have a very good next couple of months."

Jese Rodriguez, who scored the winner against Espanyol in a 1-0 win in midweek, is expected to keep his place in the absence of Bale.

And there is some good news on the injury front as French defender Raphael Varane returns to the squad for the first time since November.