Karacan picked up the injury during the 1-0 defeat at Liverpool last weekend after he twisted his knee in a challenge with Steven Gerrard.
The 23-year-old underwent scans during the week and was forced to sit out Saturday's thrilling 3-3 draw with Fulham at the Madejski Stadium.
He also 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 tonight.
"Thanks for all of your messages and support, means alot & ill be back on the pitch as soon as i can! thanks again."