Knee injury rules out Karacan
The English-born Turkey Under-21 international only recently returned to action after four months out with an ankle problem.
Karacan confirmed to fans on his Twitter page that he will face time out, and thanked them for the support they have given him.
"For those asking looks like i'm going to be out for around 6 weeks. Gutted to be injured again but could have been worse," he tweeted.
Then he added: "Thanks for all of your messages and support, means a lot & I'll be back on the pitch as soon as I can! Thanks again."
Karacan's enforced absence is a blow for Reading ahead of what could be a defining month in their season.
Brian McDermott's side face his old club Arsenal in the League Cup on Tuesday, then tackle a run of Premier League fixtures against teams who, apart from Everton, currently occupy the bottom six places in the table.
The Royals, promoted as champions from the second tier last season, are 18th in the top flight following their thrilling 3-3 draw at home to Fulham on Saturday, but still without a win after eight League games.