The Austrian picked up the problem late on in last Saturday's home loss to Luton and after being assessed by the medical staff, it has been confirmed that he is out for an extended spell.
Fellow summer signing Brad Collins will be handed his Championship bow in Saturday's key Roses test at Wigan Athletic, with assistant head coach Christopher Stern also confirming that defender Bambo Diaby and striker Cauley Woodrow will miss the trip to Lancashire.
Diaby hobbled off with an ankle problem midway of the second half in the 3-1 home setback to the Hatters, while Woodrow has missed the Reds' last two matches with a calf issue.
Offering an injury update, Stern said: "He (Radlinger) was pretty down after Saturday's match. It is a bigger injury (than we thought) and we think he is going to be out until December, so it will take a while.
"It is a ligament injury in his knee which will put him out for a while, unfortunately.
"Diaby had a scan at the start of the week and is going to be out for the Wigan game and we hope we can get him back after the international break. We will see how the next two or three weeks will go.
"Woodrow will not be available (for Wigan). Sometimes, short injuries turn out to be bigger injuries and this is unfortunately the case. But we confident in his recovery during the international break and that he will be back afterwards."
Meanwhile, Stern said it was not for him to answer as to whether Dimitri Cavare has a future of the club - with the defender having being involved in the club's last four matches.
Cavare has not featured since the 2-0 loss at Sheffield Wednesday on August 10, with social media speculation having recently focused on whether he has a future at the club.