The Turkish midfielder had already missed the start of the season as he recovered from a broken ankle that he suffered in June.
"I think I've opened up the medial ligaments a bit. When I did it I knew it wasn't great, but I was able to walk so I thought I could give it a go, but it was too loose. I'll know more this week. It's one of those things, but I've been here before. I'm hoping it won't be longer than a month," he told Sky Sports News.
"But I can't put any timescale on it until the scan. After breaking my ankle it's gutting - the worst runs through your mind. I eat, breathe and sleep football and knowing that will be taken away from me is the worst thing in the world."
Karacan made 37 appearances for the Royals last season.