The Ache
Spoilers for the most recent chapters. Based on 819.
It's not that Zoro misses Sanji. That would be unthinkable and anyway he doesn't. There's nothing Sanji can do that he can't handle on his own and better. So there is absolutely no missing Sanji here at all, not in any fiber of his body.
And yet… there is still something there.A sensation just to his right side, like he is missing an earring or to his left that the heft of his swords isn't as heavy as it should be. Like something is gone that should be there. A hollowed out space that doesn't ache exactly, or if it does, faintly, like a wound that is just almost ready to close but still pulses its presence if he moves the wrong way.
So it's not that he misses him. He's just used to a Sanji here, where he should be, or at least around… and not at some unknown location facing unknown odds, and with a yonko to boot. Not to mention there's a girl involved and that always makes the stupid ero cook even dumber than normal. Sanji will sacrifice himself for love and Zoro knows this.
So even when he folds his arms and reminds everyone of his own strength, which is after all superior to Sanji's because (among other things) he doesn't flip his shit over a pair of long eyelashes, he can't help but feel the annoying tug on his gut. The danger he can't quite meet. The strength he can't quite surmount. Not being there when he should. And the ever present prickling knowledge of that asshole being so far away, when he should be here in their midst like he belongs.