Bucky, on the other hand, is dealing with his own host of issues. She clearly doesn't recognize him in the least, let alone know what happened with the shield, with- everything. It hurts. He's been missing Natasha, his Natasha, almost since he arrived, since he learned it would be a while before they could - or would - send him back to New York City. This isn't her, it's become quite clear. This means, if she is Natasha Romanoff, she's another timeline's version, and could definitely go toe-to-toe with him if this goes pear-shaped.

"I'm Bucky Barnes. In my timeline, I took over as Cap when Steve was indisposed. When he got back, he gave me his blessing." That's the long and short of it, without going into his own grief when Steve 'died', or how much he had needed this grounding thing to keep him steady and stable. He pauses for a moment, quieter, and says, "You didn't recognize me at all, did you?"