From my understanding from Insider forums I belong to is that it's something he could have tried to play through but would have been very difficult. After Zuriah Fisher was designated as having a season ending injury two seasons ago only to somehow be able to get back on the field at the end of the season Franklin has changed how he designates injuries. Long Term is just another way of saying "season ending" but leaving the door open for a miraculous return later in the season like Fisher. I think with KJ the fact that we could potentially make a run in the playoffs and that it's far enough down the road from the injury they are potentially leaving the door open for him to return if they can get to the second round of the playoffs, or if they get a favorable first round game at home.
I have not heard that he has had surgery which was rumored to be needed for a full recovery so if surgery is indeed needed and he hasn't had it yet then I would say they are leaving the door open for him to potentially come back in a playoff scenario. If he has had surgery I'd say he will not be back this season as that usually means a longer recovery timeline.
This is all just speculation on my part so please take it with the largest grain of salt you can find.