I don't mean to disparage the Red Army. I wouldn't be asking if I thought it was an incompetent fighting force. But apparently 75% of the million casualties during that battle were Soviet and it confuses me.
The Soviets had defensive positions with land mines and other obstacles, good tanks and other weapons, by 1943 their troops were much better trained and equipped than in 1941, much more artillery and other firepower, and the Wehrmacht didn't achieve much of a breakthrough.
Fundamentally, they were still going to be on the receiving end of the fist. It wasn't that the Germans drove headlong into an immovable wall, like Pickett's Charge, but they were still able to launch a major attack with some of their best troops and equipment at the time and place of their choosing. The Soviets had to reel in order to gain time to react and send their reserve forces to meet the attack, and the folks at the receiving end of the initial strike weren't going to have a good day.
Then, when the major counter attacking forces hit the Germans, it was more of a meeting engagement than a defense. And, better though the Soviet army were at the time, they were still facing off against some pretty reasonable German forces.