Yuya just doesn't even acknowledge Nightshade's desire for the changeling--he doesn't feel like telling her 'no' is going to go well, but... the answer is 'no'.

"We do know how to use it! And we'll make sure everyone can go home--I promise. But if you get in our way, it's only going to be..." His gaze slowly drifts aside to Peridot as she yells in Steven's defense. She's right, but he really hopes that doesn't piss Nightshade off.