Hulk practiced for less than 15 minutes before leaving the field. He said the injury wasn't serious and "hopefully" it wouldn't keep him from playing in the next match.
"I felt a small discomfort and decided to leave as a precaution," Hulk said. "I was already thinking about the next match, it's only two days away and in the World Cup you don't want to miss any match. I hope to be ready so I can help the national team again."
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.
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"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."
"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.
Coach Luiz Felipe Scolari ended the training session after the hard contact between Luiz and Gustavo.