In his current condition, he was completely incapable of defeating Karna. That was beyond doubt. Karna was one of the Golden Triad, an Anomalous Servant. He had once been Shirou's idol; even Shirou's Hearthstone ID used Karna's name.
Meanwhile, Shirou was just a half-baked bounty hunter. He could deal with magi easily enough, but even soloing one of the Twenty-Seven Dead Apostle Ancestors had taken everything he had. If Karna and Shirou switched places, Karna could confidently declare that he would crack the White-Winged Duke's skull within three moves.
"Hey, Ruler. Though I expected this, I still want to ask: you won't interfere in the battle between me and the Red Faction's Lancer, even though he just tried to kill you. Is that right?"
"That is correct. As the arbiter of the Holy Grail War, I have an obligation to ensure the competition's fairness." Good. Just as expected of Jeanne Fair. Don't let me catch you favoring anyone.
"Then I have no choice. Looks like I can't take any shortcuts. Come on, Red Faction Lancer. Though I probably can't beat you, there are some things you won't know until you've fought." Rather than wait passively, Shirou chose to strike first. He used Resonance Turn to appear beside Karna, opening with a Cero entanglement. The ultimate move that could bat a True Ancestor around like a baseball was effortlessly caught by Karna.
"That was not what you truly meant," Karna said. "From the very beginning, you never intended to borrow Ruler's strength."
It was said that Karna had once asked the poor about the meaning of life at the ends of the earth. After his death, that deed had been elevated into one of his skills: Discernment of the Poor B+. With that ability, Karna was practically a human lie detector.