Bale remains a doubt for Atletico Madrid clash
The Wales international limped off five minutes into his team's La Liga meeting with Malaga on Saturday, with the reigning European champions going on to win 3-1.
Real and Atleti are tied at 0-0 heading into their second leg at the Santiago Bernabeu on Wednesday, but Bale's involvement hangs in the balance.
A club statement on Monday read: "Following tests carried out on Gareth Bale at the Sanitas La Moraleja Hospital by the Real Madrid Medical Services, he has been diagnosed with a muscle injury in his left soleus.
"His progress will be monitored."