I think you would need a major POD back in early 1943 in order for Manstein to make any sort of longer term difference; say that after his triumph in the Kharkov counteroffensive he is appointed to de facto supreme command of all German forces in the east, with broad authority to run a sane strategy of flexible defense all along the front (but NOT to mount his mooted Backhanded Blow offensive, which would likely have been almost as disastrous as Kursk), I can see the war lasting a few months longer, heavier Soviet casualties, and hostilities ending with the western Allies taking Berlin and Anglo-American forces occupying all Germany west of the Oder.