Based on this piece of information provided by

a Russian.

"On 20 December three tank regiments (52, 128, 223) arrived to the Stalingrad Front together with the 2nd Guards Army."

It is safe to say that the 2nd Guards Army was away from the frontline on 12/19/1942.  Like I said, the Sixth Army had to break through the defensive line set up by the Russian 57th Army to link to the rescue forces.

The rescue forces were facing the Russian 51st Army btw. 

If you understand Russian, this map should be a good reference to the situation:

http://battle.volgadmin.ru/karta/25.htm