The 25-year-old centre-back, who headed Germany's second goal in their 4-0 win over Portugal on Monday, is struggling to be fit for the Group G clash in Fortaleza.
He has been unable to train with the squad since their opening win in Salvador, when he had to be helped off with deep bruising to his right thigh in the second half.
He followed an individual programme on Thursday and is being treated intensely.
Should the Borussia Dortmund star miss out, right-back Jerome Boateng is expected to move across to partner Per Mertesacker at centre-back.
Sampdoria's 22-year-old Shkodran Mustafi will win his third cap at right-back having come on for Hummels in Salvador.
Arsenal defender Mertesacker told Thursday's press conference that "he assumes Mats can play on Saturday", but a German FA spokesman said there was no prognosis as to when Hummels will be fit.