“I didn't put my name in that goblet!” said Harry, starting to feel angry.
“Yeah, okay,” said Ron, in exactly the same sceptical tone as Cedric. “Only you said this morning you'd have done it last night, and no one would've seen you.…I'm not stupid, you know.”
“You're doing a really good impression of it,” Harry snapped.
“Yeah?” said Ron, ...
I don't quite understand the use of 'only' here. Is it modifying "this morning", 'you' or something else? What does it truly convey?