"After the tests carried out on Gareth Bale by the Real Madrid medical team, a muscular injury has been diagnosed in his right pyramidalis muscle," the club said in a statement on Monday without stating how long they expect Bale to be sidelined.
The pyramidalis is a small muscle of the anterior abdominal wall.
The 25-year-old was an unused substitute in Madrid's 5-0 thrashing of Levante on Saturday.
And Real boss Carlo Ancelotti insisted after that game he wouldn't take any risks with Bale's fitness given his side face an even more demanding task when Barcelona visit the Santiago Bernabeu on Saturday.
However, Madrid sports daily AS reported on Monday that Bale will also miss the first El Clásico of the season, as the Real Madrid–Barcelona games are known.
The European champions sit comfortably on top of Group B with maximum points from their opening two games, three points ahead of Liverpool and Basel.