That it would fail to achieve the result Manstein desired (A repeat of Third Kharkov on an even grander scale) and result in German losses equivalent to or greater than OTL.
The Soviets also would suffer great losses as they did at 4th Kharkov which is probably a good guide to the kind of fighting the Backhand blow would have seen. Some territory might have been permanently retaken by the Soviets. They would have taken very high losses but it would have been costly for the Germans too.
Whether it would make a difference in the long term depends on whether Stalin,in the wake of a costly failed summer offensive in 1943 would have been willing to offer a seperate peace and whether Hitler would be prepared to agree to that. If so it might be that some territory has to be returned to the Soviets or the ceasefire line reflects the military situation at the time. This allows the Germans to transfer a significant portion of their forces West to effectively prevent an Anglo American invasion of Italy let alone NW Europe.
At this time there is no gauruntee that the atomic bomb program will work so this could well effectively be the end of WW2 With the Soviet Union out of the war the only strategy for the Western allies at this point would be blockade and strategic bombing with no end in site and certainly little hope of anything other than a stalemate.
After that the best the Western Allies can hope for is that Hitler will agree to withdraw from France, Belgium, Holland and Norway.Denmark an Poland will be annexed to the Third Reich. The Germans probably would withdraw from Greece and Yugoslavia.