"X-rays taken in a Kiev clinic have revealed depressing news - a double break of the right tibia," Shakhtar said on their website.
"The treatment may take 10 weeks followed by difficult rehabilitation."
The Brazil international broke his leg in Friday's 1-0 defeat by Obolon Kiev.
"Unfortunately Fernandinho is out for the whole season," Shakhtar coach Mircea Lucescu told local media.
Shakhtar start their Champions League campaign at home to Serbian champions Partizan Belgrade on Wednesday.
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