Uranus isn't assured if Stalingrad falls... part of the reason it was so successful was that the two strongest German formations for 300 miles (the 6th army and the 4th panzer army) where tied down on only 50 miles of front... if the city is conquered, then they can adopt more regular levels of divisional spacing and corset stiffen the satellite divisions
It might work if the Soviets call off Mars, is this possible? Although it probably wouldn't be as successful.