I was just reading Manstein's "Lost Victories", and i came across some interesting information about the subject of this thread.
"The moment Sixth Army launched its attack towards the south-west things would become easier for Fourth Panzer Army, whose further progress over the Mishkova was by no means a certainty on 19th December, would doubtless be able to accomplish the outstanding 12 miles once the pressure were relieved by the action of Sixth Army"
So Manstein thought it was still possible for Sixth Army to link up with Forth Panzer Army and establish enough contact to provide Sixth Army with the fuel, ammunition and food it needed for continuing its breakthrough.