Arguably the high water mark of the German eastern front campaign was the capture of Sevastopol by General Erich von Manstein, who was arguably Germany's best general. After this victory, he and his 11th Army were transferred to the Leningrad front where he was expected to earn similar success.
His plan for an assault on Leningrad was forestalled by an unexpected Soviet "spoiling" attack at Sinyavino by nearly 200,000 troops that had escaped the attention of German military intelligence. The effect of one extra army for each side was a draw, rather than a victory for one side or another.
William L. Shirer in the "Rise and Fall of the Third Reich" and Walter Kerr in "The Secret of Stalingrad" allege that Germany military intelligence underestimated the available 1942 Soviet troop strength by 1-2 million men. Some of these men perhaps formed part of the Sinyavino force, and many of them found their way to Stalingrad (during the siege) or the Caucasus.
Do historians believe that the Germans were badly informed about Soviet force strength around Leningrad, and that only the "fortuitous" transfer of Manstein's 11th army allowed for a German "draw" rather than a loss (the breaking of the Leningrad siege and the rolling back of Army Group North)?
Are there historians who argue that the Germans underestimated Soviet strength in the south to such a degree that even if Manstein's 12 divisions had been sent to the Caucasus (or to relieve Stalingrad) that they would have been a "drop in the bucket" against the actual Soviet forces (as opposed to the ones that the Germans had identified)?