He's never been good for asking for help. It's especially a problem when he's injured. You've noticed this, because you've noticed everything about all your crewmates over the years. And Sanji's behavior always catches your eyes. Some of his actions aren't not concerning, but at least they're not dangerous. But he has a tendency to avoid medical care. He doesn't fight it like Zoro or pout about it like Luffy, but he avoids Chopper's attention after battle with professional skill.

And honestly, he doesn't need medical intervention now, not really, not with how fast he heals. But still, it would speed up the process and afford him less pain if he would just ask for help. So, when one faithful afternoon you catch him holding a healing wound in pain, you confront him.