The 25-year-old suffered a ligament injury in the pre-season match against Haugesund in Norway and it was hoped he would be back in full training within a few days.
However, Kevin Blackwell told the club's official website: "It's a little bit worse than we thought. After having a scan it showed the ligament damage is a bit worse and so he might take a little bit longer to recover than we initially thought.
"I wouldn't rule him out for the start of the season or anything like that though just yet. But at the same time we won't take any risks and rush him back."