The 22-year-old Frenchman suffered ligament damage during Saturday's 1-0 win over Middlesbrough forcing him to miss Tuesday night's home defeat by Brighton.
"Souleymane has ruptured his ankle ligaments," boss Dave Hockaday told the club's official website. "He was involved in a really bad tackle against Middlesbrough right on half-time.
"It was right in front of the dug-out and we heard the contact and it looks like he is going to be out for weeks.
"It won't just be for one or two weeks, he will be out for a number of weeks. We'll need to see how it settles down but I think it will be three or four weeks minimum.
"It is a blow and he is a big miss for us. We need more pace and Souley gives us that pace and a bit of power."