The Wales forward, 25, was substituted with the problem after just five minutes of
Real's 3-1 home victory over Malaga on Saturday.
"Bale has been diagnosed with a muscular injury in his left soleus. His condition will be evaluated," his club said in a statement on Monday.
The
first leg at Atletico ended 0-0.
Los Merengues are also without Croatia midfielder Luka Modric, who suffered a knee ligament strain against Malaga and is expected to be unavailable for six weeks.
However, Real coach Carlo Ancelotti is hoping France striker Karim Benzema will return after missing Saturday's win at the Bernabeu with a knee injury.
Brazil left-back Marcelo is also suspended for the home side, meaning Portugal international Fabio Coentrao will start.
Atletico coach Diego Simone welcomed Croatia striker Mario Mandzukic back to training on Monday, after he missed Saturday's win at Deportivo La Coruna with an ankle problem.
But Los Rojiblancos will be without suspended midfielder Mario Suarez against their neighbours in a repeat of last year's final, which they lost.
