The 23-year-old underwent scans this week after he sustained the injury during the 1-0 defeat at Liverpool seven days ago.
Karacan missed the start of the season as he battled back from a four-month lay-off due to an ankle injury and he faces yet more time out.
"For those asking looks like im (sic) going to be out for around 6 weeks... gutted to be injured again but could have been worse," he wrote on his Twitter page.
"Thanks for all of your messages and support, means alot & ill be back on the pitch as soon as i can! thanks again."