Gonzalo Higuain suffered an ankle injury and left the pitch on a stretcher during Napoli's 0-0 draw at Inter on Saturday.
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The Argentine striker was hurt as he took a shot in the final minutes at San Siro.
Marco Andreolli blocked the shot, and Higuain kicked the Inter defender's shin on his follow through.
"Gonzalo Higuain has a trauma to the right ankle, which will be evaluated over the next few days," the club said in a statement.
Napoli manager Rafa Benitez labelled the injury a "bruise" after the match.
"Higuain has a trauma to the ankle, a bruise and we’ll have to see over the next few days," Benitez said. "We still don't know what the situation is."
Napoli play in the Coppa Italia final against Fiorentina next Saturday, while the World Cup is seven weeks away.
"We are all worried about him and hope it's nothing serious," said Napoli midfielder Gokhan Inler. "Higuain is very important to us."