Near the end of the story, a drunken Mel uses the word “vessel” when he means to say “vassal,” and Terri corrects him. In his passive-aggressive response to his wife’s correction, Mel metaphorically refers to himself as “just a mechanic,” using this metaphor as a form of verbal irony:
“All right,” Mel said. “So I’m not educated. I learned my stuff, I’m a heart surgeon, sure, but I’m just a mechanic. I go in and fuck around and fix things. Shit,” Mel said.
“Modesty doesn’t become you,” Terri said.
“He’s just a humble sawbones,” I said.
When Mel metaphorically refers to himself as “just a mechanic,” he is using verbal irony, saying that as a surgeon he merely “fuck[s] around and fix[es] things” inside a patient’s body in the same way that a mechanic tinkers with the parts inside of a car. Both Terri and Nick (the narrator) notice here that Mel—who went through medical school and rigorous surgical training—is being intentionally (and disingenuously) self-disparaging, with Terri telling Mel that “modesty doesn’t become [him]” and Nick sarcastically calling Mel a “humble sawbones.”
It is notable that, with the character of Mel, Carver is suggesting, in some ways, that he is just a mechanic. As a cardiologist, Mel understands the human heart in a practical and rational way, but not in a deeper or more emotional way. He has been talking about love all night but doesn’t seem any closer to expressing—or embodying—it, as seen in his passive-aggressive response to his wife and his mistreatment of her throughout the story.