The Borussia Dortmund defender limped off in a 4-0 Group G victory at Arena Fonte Nova in Salvador, having earlier scored Germany's second goal.
Thomas Muller helped himself to a hat-trick as Joachim Low's side capitalised on Pepe's red card in the first half to start the tournament in style, but there was concern over Hummels.
The 25-year-old is confident his knee injury is not serious, though, and is hopeful of lining up against Ghana on Saturday before a reunion with former Germany coach Jurgen Klinsmann and his United States side five days later.
"For me, it's just a knock, and I hope it stays that way," he told ARD.
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