Carvajal, who recently signed a new long-term contract with Los Blancos, was found to have "a viral pericardium infection", which is a condition affecting the membrane enclosing the heart.
According to Marca, the 25-year-old will miss 'a minimum of two months' as he attempts to recover from a condition which has been described as 'rare and difficult to treat'.
The Spain international was an ever-present for Madrid in the early weeks of the 2017-18 campaign, but there are now question marks over whether he will return to action before the end of the year.
"Following tests carried out on Dani Carvajal, he has been diagnosed with a viral pericardium infection. His recovery will continue to be assessed," read Madrid's statement last weekend.