from link:
This is not correct as far as I know. After Rostov fell, Hitler gave priority to the capture of the oilfields, so in fact the force (6th Army) that was tasked with the capture of Stalingrad was denuded of men and supplies. Arguably, had it been at full strength it would have taken the city by storm initially. In any case, it was not necessary for the Germans to occupy the entire city, once they had reached the shores of the Volga their strategic aims were fullfilled as they could cut river traffic feeding supplies to the Red Army in the south.