Hulk said he had the injury before the opening win over Croatia, when he was replaced late in the match, and felt it again after a few shots on goal on Sunday.
He walked off the field without a limp, but then sat down on a bench and lowered his head in apparent dismay. Hulk, who plays for Zenit in the Russian league, put his hand on his left leg when team medical staff arrived to treat him.
A few moments later he went to the locker room accompanied by one of the team's doctors.
"I was feeling down when it happened because you always think about the worst," Hulk said. "But then I talked to the doctors and they told me that it wasn't anything too serious. I'm going to undergo treatment and hopefully I'll be able to play."
The Brazilian federation said Hulk wouldn't need additional tests and would travel as planned with the rest of the squad to the northeastern city of Fortaleza later Sunday.
"At a World Cup you have to overcome every challenge," Hulk said. "We prepare our entire lives to fulfill our dreams of playing in the tournament. I'm feeling well."
If Hulk is ruled out, 21-year-old Shakhtar Donetsk striker Bernard would be his likely replacement. Chelsea's Ramires is also an option.
Brazil had been injury free since it started its World Cup preparations in late May, but Sunday was a tense practice session in Teresopolis, outside Rio de Janeiro.
Bernard stayed on the ground for a few moments after a foul by David Luiz, and the defender was later hurt in a challenge by midfielder Luiz Gustavo. Both players were down briefly but recovered.
Coach Luiz Felipe Scolari ended the training session after the hard contact between Luiz and Gustavo.
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