Zlatan Ibrahimovic left Paris Saint-Germain's match early in the first half with a back injury on Saturday.
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Ibrahimovic was on the ball trying to shake a defender when he was injured in the opening stages of PSG's 2-0 win against Bastia.
The Swedish striker grabbed his left side and tried to continue, but he was eventually replaced by Ezequiel Lavezzi in the 16th minute.
PSG boss Laurent Blanc said after the match that he did not know how serious Ibrahimovic's injury might be.
"At first I thought he had taken a knock, but no, it was as he was turning that he hurt himself in the side. It is a strange injury," Blanc said.
"I am not a doctor. I don't know if it is serious, but knowing Ibra, for him not to finish the match the pain must have been quite intense. We will need to see with the medical staff if it is serious or not."
The injury is the second to one of PSG's star players this week after Thiago Silva left a friendly on Monday with a thigh injury. Silva will be sidelined for "several weeks," the club said.
Goals from Lucas Moura and Edinson Cavani against Bastia helped the French champions get over last week's disappointing draw with Reims.