But coach Carlo Ancelotti is confident the problem is not serious.
Bale will join the suspended Cristiano Ronaldo on the sidelines for Wednesday's match as Madrid seek victory to secure passage to the quarter-finals after a 0-0 draw in the away leg.
"Bale has taken a knock to the calf," Ancelotti said. "We will see how it evolves in the coming hours but I do not think it is a problem for Sunday."
Madrid, five points behind Barcelona and Atletico Madrid in La Liga, travel to Valencia at the weekend.
Madrid are coming off a 2-2 draw at Osasuna which saw them lose further ground on their rivals, but Ancelotti is not worried.
"I don't think Osasuna was a bad game," he said. "The first 20 minutes were the best by the team this season. We must continue and be confident."