The 25-year-old was Nigel Clough's first signing when he arrived from Motherwell in January but has struggled with the injury and has been unable to get a run in the side.
Porter missed the final two months of last season and the problem flared up again in pre-season.
"Everything went according to plan and he will be out until Christmas," said Clough after his side's 2-1 Carling Cup defeat to Rotherham at the Don Valley Stadium.
"It is a blow for him and for us. We are not likely to see him until at least Christmas but Chris is a great lad and he will work hard on his rehab."