It's the same if Zlatan scores a goal in CL against a big team and some people laughing at those who called him a big game flop, using that single match as an argument that Zlatan isn't a flop against the big teams in important matches.
But great match with a goal won't mean that Zlatan wasn't a big match flop so far, just like an awful match against tonight's Juve doesn't mean that Cassano wasn't absolutely amazing so far.
Sampdoria could have had Messi in Cassano's place tonight and nothing would have changed because Messi wasn't gonna get the ball either with our DMCs intercepting everything and Chiellini playing a monster match.
I agree though that it was premature saying that Cassano and Sampdoria will destroy us. Even if we played bad and they played great recently, you can never claim that someone will destroy Juve in Turin. Even less when it's Sampdoria we're talking about.