The winger was substituted during the Gunners' 3-0 win over Bolton Wanderers at the Emirates Stadium, and Wenger has said that his original diagnosis was wrong.
"I don't really know what happened with Theo," said the Frenchman. "I thought it was a classic hamstring injury but it is not.
"He felt a sharp pain in his knee. I don't know what the diagnosis will be but it was completely surprising. I don't know if he will have a scan tonight. I will have to speak to the medical staff."